Diurnal changes in plant and nodule performance were studied in 28–9 d plants of Pisum sativum L. in two environments, both with a 12 h (27 000 lx):12 h::light:dark cycle, but one (A) with a fluctuating temperature-humidity regime (photoperiod 18 °C, 60 per cent relative humidity:night 12 °C, 85 per cent), the other (B) with constant temperature (18 °C) and humidity (75 per cent). Fixation rate (C2H2 reduction), respiratory output of the nodulated root, and nodule sugar level increased throughout the photoperiod, whereas nodule soluble nitrogen level declined steadily. Reversal of these trends in the night period led, at its end, to minima in fixation rate, sugar level and respiration, but a maximum in soluble nitrogen. The A environment produced the greater day:night fluctuations in transpiration and nodule soluble nitrogen, but B, with its higher night temperature, induced the more pronounced decrease in fixation at night. Slightly less nitrogen was fixed during the photoperiod than during the night in the A environment, yet since some fixation products were retained in the nodules at night and not released until the next photoperiod, the day: night difference in nitrogen export from nodules was 1.8:1. The photoperiod of A was also a time of higher nodule respiration and replenishment of nodule sugar and starch, so that the nodules' requirement for translocated carbohydrate was more than twice that at night. Humidity decrease in the photoperiod (of A) elicited higher rates of transpiration and a more rapid than normal emptying of soluble nitrogen from the nodules: elevation of humidity had the opposite effects. Shoot removal (A-grown plants) caused nodule sugar levels to fall rapidly below those normally encountered in intact plants.