SUMMARY An investigation of the effect of light on the diurnal rhythms of urine and electrolyte excretion has been carried out on six healthy male subjects living under strictly controlled conditions. The light reception usually associated with waking was delayed by keeping the subject in darkness for 3 hr while the normal activity routine was maintained. The normal morning increase in urine and electrolyte excretion was much lessened by the absence of light and its effects were apparent for the rest of the day. Light plays a large part in the causation and timing of the diurnal rhythms of urine and electrolyte excretion.