Squash (Cucurbita pepo), cucumber (Cucumis sativus) and sweet melon (Cucumis dudain) plants were grown in sand culture with N as the variable and were harvested at the early flowering stage. The plants at this time showed a definite gradation of symptoms from severe to no deficiency of N. The tops were separated into leaves and stems. Leaves were separated into young, mature and old and then subdivided respectively into petioles and blades. The petioles were analyzed for NO3‐N. The critical NO3‐N concentration for squash, cucumber and melon on a dry basis was 1000, 2000 and 3000 ppm, respectively. A relatively high concentration of NO3‐N in the nutrient solution decreased the growth of squash and cucumber plants significantly, but had no effect on melon plants. Melon plants can tolerate relatively high concentrations of N0,‐N in the plant tissues, while squash and cucumber cannot.