Adult rams were subjected to various photoregimes for more than 4 years to determine whether pituitary and testicular activities are inherently cyclic. Adult rams (three treatment groups, each with 12 rams) of the Arcott breeds were housed in environmental rooms kept at 15°C and exposed to short days (8 h of light and 16 h of darkness) for 1 year to dissipate the possible influence of normal environmental changes. The rams were then exposed to long days (16 h of light and 8 h of darkness) for 3 months to induce testicular regression. Subsequently, rams in Treatment Group 1 were exposed to short days for 39 months. Rams in the other treatment groups were exposed either to alternating 4-month periods of short days and long days for 16 months followed by 21 months of short days (Treatment Group 2), or to alternating 4 month periods of short days and 2 month periods of long days for 12 months followed by 4 months of short days, and then a reversal in daylength to 21 months of long days (Treatment Group 3). Exposure to the fixed daylength photoregime was followed with a change in daylength to test the retention of photosensitivity in the rams. Scrotal circumference and serum concentrations of follicle stimulating hormone (FSH), testosterone and prolactin were measured every 2 weeks throughout the study. In the rams exposed to short days only (Treatment Group 1), no cyclic change in hormone levels or scrotal size was apparent; FSH levels, testosterone levels and scrotal size all rose significantly early in the 39 months period and remained above basal levels. In the rams of Treatment Groups 2 and 3, the changes in the photoperiod cycles induced rhythmic pituitary and testicular activities which persisted for the first year of constant daylength, faded or disappeared thereafter. Prolactin levels in each treatment depended on the light regime, with levels being depressed or elevated during short or long days, respectively ( P < 0.01). All rams responded to the final change in photoperiod with an appropriate increase or decrease in the pituitary and testicular activities. These observations indicate that persistent memory of previous photoperiod change accounts for the `apparently spontaneous' rhythms in pituitary and testicular activities in the short term, but the decline of the regular changes as the memory fades does not support the existence of inherent cyclicity in reproductive functions in rams of the Arcott breeds. 5 Authors: G.A. Langford, L.M. Sanford, G.J. Marcus and J.N.B. Shrestha. For the Department of Agriculture and Agri-Food Canada, Government of Canada, © Minister of Public Works and Government Services Canada (1998). 5