Introduction. Studies of affect structure published in the last decade consistently suggest two general factors or dominant dimensions, usually called positive affect and negative affect. Positive affect is a dimension of enthusiasm, activation and alert, High positive affect is a state of high energy, full concentration and pleasurable engagement, whereas low positive affect is characterized by sadness and lethargy. Negative affect is a general dimension of subjective distress and unpleasurable engagement that subsumes a variety of aversive mood states (like anger, contempt, disgust, fear and nervousness). Low negative affect is a state of calmness and serenity. Numerous Positive and negative affect scales have been development and studied. One of them, the Positive and Negative Affect Schedule:. PANAS (Watson, Clark y Tellegen, 1988) have demonstrated that is one of the most reliable, valid and efficient means for measuring these dominant dimensions of mood. The PANAS consist of two 10-items scales that: a) Are internally consistent (aphas = 0.86 - 0.90 for positive affect and 0.84 - 0.87 for negative affect), b) Have excellent convergent discriminant correlations with lengthier measures of the underlying mood factors, c) Have demonstrated appropriate stability over a 2-month time period, d) Correlate at predicted levels with measures of related constructs, and show the same pattern of relations with external variables (e.g. "social activity" correlated with positive affect and not with negative affect, or "perceived stress" correlated with negative affect and not with positive affect). PANAS has been translated into German, and a children version was developed (PANAS-C). These versions also demonstrated appropriate psychometric characteristics, The aim of this study was to translate into Spanish and evaluate the psychometric properties of a Spanish version of positive and negative affect scales (PANAS) in a Mexican population. Method: Three studies were performed. After the translation processes was performed, internal consistency and factor analysis were analyzed in a 321 sample of college students and their families and friends. In the second study, test-retest reliability was determined in 188 subjects (college students and their families and friends), with a 2 week administration interval. In the third study the external validity was determined correlating PANAS with Beck's Anxiety and Depression Inventories. Beck Depression Inventory is a 21-item self-report measure of depressive symptomatology, and Beck Anxiety Inventory is a 21-item self report measure of anxiety symptomatology. The psychometric studies of both measures have generally supported their validity and reliability and their use in clinical and research contexts. Results: Fist study: Internal consistency and Factorial Validity The first study sample included 321 subjects, equally divided between men and woman (51.4% females, and 48.6% males), with a mean age of 35.5 ± 14.1 years and 15.8 ± 3.5 years of education (range= 2 - 28 years). The mean for positive affect was 33.5 ± 7.4 and for negative affect was 20.1 ± 7.1. Internal consistency reliabilities were calculated for two different point-time instructions in both positive and negative scales: 1) "last week" and "generally". Cronbach's alpha coefficients were: a) for positive affect scales (last week and generally) = 0.85 - 0.90, and for negative affect scales (last week and generally) = 0.81 - 0.85. Finally, the factor structure coincided with the English version one. Two dominant factors emerged. Second Study: Test Retest reliability The sample for the second study included 188 subjects, 97 (51.6%) females and 91 (48.4%) males. Mean age were 38.5 ± 14.4 years (range = 16 - 80 years). The intra class correlation coefficient for positive affect was 0.34 - 0.32 and for negative affect 0.31 - 0.42. Third Study: Correlation with depression and anxiety measures The third study were performed with a 32 subjects, 65 (49.20%) males and y 67 (50.8%) females, with a mean age of 31.40 ± 12.15 (range = 17-73 years) and 15.50 ± 3.51 years of education. Coefficients ranging 0.32 and 0.55 were obtained. Discussion: Spanish version of positive and negative affect scales (PANAS) show similar psychometric properties to its English counterpart. Positive and negative scales in both point.-time instructions ("last week" and generally") demonstrated high internal coefficient levels. The factor structured was the expected one. Two factors were obtained. They also demonstrate appropriate stability over a 2 weeks time period for the "last week" instruction. We concluded that these Spanish version of the PANAS scales are valid and reliability measures for measuring the dimensions of mood, and also suggest to evaluate them with other point-time instructions.