It is well established that the time of ovulation in the rat is related to the environmental light/dark cycle (Everett, 1948). Rats also show a diurnal rhythm of spontaneous running, and it seemed a theoretical possibility that the effect of light on the luteinizing hormone (LH)-release mechanism might be mediated via general bodily activity. As a first step in testing this hypothesis, the relationship of ovulation time to the activity cycle was investigated under conditions of constant darkness, both activity patterns and oestrous cycles having first been disrupted by prolonged exposure to constant light. Twelve regularly cyclic control rats were exposed to 14 h light and 10 h darkness daily, and were kept for at least 8 days in individual cages, each having an activity wheel attached. The number of revolutions/h was recorded by means of a Miniscript recorder. Daily vaginal smears were taken, and on a day of oestrus