The functional load (FL) of a phonological contrast has been shown to correlate with its resistance to merger on evolutionary timescales. The effects of FL on day-today speech, however, remain an uncharted territory. In this paper, we studied the effects of FL on the production and perception of vowel length contrasts in Bangkok Thai. We found that, in production, FL had a positive correlation with long/short vowel duration ratios, as well as with the discriminability between short and long vowels distributions in duration space. For perception, we found no correlations between FL and any of the perceptual measurements. We hypothesize that different units and mechanisms involved in production and perception, as well as their relationship to phonological contrast, are responsible for the presence or lack of effects of FL. We also discuss the implications of our findings for theories of sound change that privilege perception over production. Finally, we show how the real-time effects of FL on vowel length contrasts production may be accommodated in a non-linear dynamical model of phonological contrast.