The effect of application of different potassium rates on some parameters of nitrate metabolism and yield in cucumber plants (Cucumis sativus) was studied. All plants were grown under controlled conditions in an experimental greenhouse. The treatments consisted of applications of K+ at three rates in the form of K2SO4 (Kl: 0.075 mg ml−1, K2: 0.15 mg ml−1, and K3: 0.30 mg ml−1). The results showed a positive effect of higher K+ fertilisation (0.30 mg ml−1) on uptake, translocation and reduction of NO3− in leaves compared with the lowest K+ rate. In addition, the higher K+ rates strengthened the translocation of organic nitrogenous compounds (amino acids) towards the fruit, thereby perhaps also enhancing the maximal commercial yield. In conclusion, for improved cucumber cultivation under greenhouse conditions, 0.15 mg ml−1 of K+ gave maximal yield, while the application of 0.30 mg ml−1 increased the metabolism and efficient utilisation of NO3−.