In the expanding leaves of Capsicum annuum L. cv. California Wonder, two of the three peaks of nitrate reductase activity associated with the light period exhibit a circadian rhythm that persists in continuous light.The spray application of kinetin to the whole shoot or to leaves other than the ones used for nitrate reductase assay causes a phase shift in the activity peaks and this has been used in preliminary investigations of the character of the mechanisms controlling the timing of the peaks.There was some indication that the rate of translocation of nitrate from the roots might be involved. The levels of nitrate moving up the stem after kinetin treatment were more dependent upon the rate of sap flow than on the concentration of nitrate in the sap. For this reason, transpiration rates in whole plants were measured after kinetin treatment but no change in pattern was seen that would correlate with the phase shift in nitrate reductase activity.The occurrence of nitrate reductase peaks in excised leaves suggested a leaf-based in addition to a root-or stem-based mechanism in the timlng of nitrate reductase activity in the leaves.In previous reports (22)(23)(24), the metabolic fate of photosynthetically fixed carbon was shown to be very closely linked in time with the activity of nitrate reductase in leaves of Capsicum annuum. Nitrate reductase exhibited three peaks of activity during and immediately after the light period and they were numbered: peak I, the peak observed soon after the end of the photoperiod that was correlated with a rise in carbon flow into amino acids during the last 2 hr of the photoperiod (22); peak II, that observed within 1 hr of the start of the photoperiod; and peak III, the major peak, observed 6 hr after the inductive lighton signal (24).The resultant periodicity in the supply of reduced nitrogen in the leaf was paralleled directly in the synthesis of amino acids from newly fixed carbon.A sequence of control mechanisms diverting carbon from the reductive pentose phosphate pathway to amino acid synthesis involved the activation of pyruvic kinase, possibly by NH4+, resulting from the activity of nitrate reductase (23). Thus, the periodic activity of leaf nitrate reductase (NO3R) seemed to play a primary role in this control sequence, and to study it further, some characteristics of two of these nitrate reductase peaks, that at the beginning of the photoperiod (peak II) and that mahifest soon after the end of the photoperiod (peak I), have been investigated. Both peaks show entrainment under constant environmental conditions and exhibit phase shifts when the plant is treated with kinetin. Some mechanisms possibly involved with the timing of nitrate reductase activity have been investigated.