The effects of long-term cold (5/3 degrees C) and salicylic acid (SA) (0.1 mM) treatments on the diurnal pattern of nitrate reductase (NR) activity in spinach (Spinacia oleracea L. cv. Gladiator) leaves were investigated. When the activities with cold treatment are compared to the control (20/15 degrees C), NR activity was decreased by 5 degrees C in the light period and by 3 degrees C in the dark period. The cold treatment also affected the timing of circadian rhythm in the diurnal pattern of NR activity. When the activities with SA treatments are compared to control and cold, it was determined that SA increases NR activity in the light period of control while SA decreases NR activity in the light period of cold.