The efficiency of nitrate assimilation by 10-day-old seedlings of three Pima, S-7, O.B, Conq, (Gossypium barbadense L.) and four Acala, Maxxa, GC-510, Royale, SJ-2, (Gossypium hirsutum L.) cotton cultivars was compared. The seedlings were induced with 0.05, 0.1 or 1.0 mM NO₃^- for 24 h. Nitrate uptake, efflux and in vivo reduction were determined. Nitrate uptake rates of Pima cultivars were about 50 percent higher when induced at 0.05 mM NO₃^- as compared to the Acala cultivars. The differences in uptake rates narrowed as NO₃^- concentration of the uptake solutions was increased. In contrast, NO₃^- efflux from roots of Pima cotton was about two-fold higher at all levels of NO₃^-. Nitrate concentration in roots induced with 1.0 mM NO₃^- was three-fold higher than in those induced with 0.05 mM NO₃^-; however NO₃^- efflux increased only slightly. In vivo NO₃^- reduction by Pima cotton was also higher than that in Acala. The results indicate that Pima cultivars are more efficient in NO₃^- assimilation at lower than at higher NO₃^- concentrations.