In vivo 31p nuclear magnetic resonance (NMR) was used to characterize the effect of the N form (NO3 vs. NH4) and the external pH (4, 6, and 8), on the intracellular pH of root tips (0-5 mm) and root segments (5-30 mm). Ammonium-grown root tips were the most sensitive to changes in the external pH. In vivo 15N NMR was used to characterize the pathway of primary ammonium assimilation in the a m m o n i u m -g r o w n roots and to compare the activity of the apical and more-basal root parts. The kinetics of 15NH4 + incorporation showed that primary assimilation in both root tips and root segments followed the glutamine synthetase (GS) pathway. In agreement with the reported gradient of GS along the seminal root of maize, incorporation of label into glutamine amide was more rapid in tips than in segments. It is suggested that this higher GS activity increases the endogenous proton production and thus contributes to the greater dependence of the cytoplasmic pH on the external pH in the ammoniumtreated root tips.