The kinetics of nitrate (NO3-) uptake by intact 23 day old lice plants was studied by measuring the depletion of NO3- in solutions flowing over the plant roots. A Michaelis-Menten kinetic model was applied allowing the uptake kinetics to be characterized by two parameters: the apparent half-velocity constant, K-m, and the apparent maximum uptake rate, V-max A propagation of uncertainty calculation revealed that the kinetic parameters could be determined with a high degree of accuracy; the standard deviation in K-m was typically 15% of the K-m value; the standard deviation in V-max was typically 7% of the V-max value. The plants were exposed to full nutrient solutions containing NO3- at 50, 200, 500 and 800 mu M for 24 h prior to kinetic testing, and both K-m and V-max were found to vary with pretreament NO3- concentration, [NO3-]; plants pretreated at high [NO3-] had lower V-max and higher K-m values than plants pretreated at lower [NO3-]. However tile variations in V,, were more consistent than those in K-m. These changes in the kinetic parameters reflect an uptake system which is capable of compensating for changes in the external [NO3-] to maintain a virtually constant NO3- uptake rate in the range studied. Changes in K-m and V-max begin within 4 h of a change in [NO3-]. Light deprivation during pretreatment in 200 mu M NO3- resulted in a complete cessation of NO3- uptake; 4-8 h of illumination were required before the uptake resumed, and uptake rates had not yet reached normal levels 8 h after the resumption of illumination.