Dark-grown, detopped corn seedlings (cv. Pioneer 3369A) were exposed to treatment solutions containing Ca(NO(3))(2), NaNO(3), or KNO(3); KNO(3) plus 50 or 100 millimolar sorbitol; and KNO(3) at root temperatures of 30, 22, or 16 C. In all experiments, the accelerated phase of NO(3) (-) transport had previously been induced by prior exposure to NO(3) (-) for 10 hours. The experimental system allowed direct measurements of net NO(3) (-) uptake and translocation, and calculation of NO(3) (-) reduction in the root. The presence of K(+) resulted in small increases in NO(3) (-) uptake, but appreciably stimulated NO(3) (-) translocation out of the root. Enhanced translocation was associated with a marked decrease in the proportion of absorbed NO(3) (-) that was reduced in the root. When translocation was slowed by osmoticum or by low root temperatures, a greater proportion of absorbed NO(3) (-) was reduced in the presence of K(+). Results support the proposition that NO(3) (-) reduction in the root is reciprocally related to the rate of NO(3) (-) transport through the root symplasm.