Two groups of 6 rams were kept under constant photoperiod consisting of short days (8 h light (L): 16 h dark (D); Group S) and long days (16 h L:8 h D; Group L) from 4 to 38 months of age. Five other rams were reared under a photoperiod representative of that occurring naturally (Group N). Testis size and plasma prolactin concentrations were obtained weekly. These data were subjected to time series analysis. The results indicated that there were persistent periodic excursions in both parameters measured. In Group N, the average cycle length for both testis volume and plasma prolactin was about 1 year and the peaks in plasma prolactin preceded those in testis volume by about 18 weeks. Rams from Group L also showed rhythmical changes in these parameters with periodicities of around 35 weeks and it is suggested that these cyclic changes may constitute true endogenous circannual rhythms; again the prolactin peaks preceded those of testis volume by about 18 weeks. Overall, rams from Group S had excursions of testis growth of a similar magnitude to those of Group L but the changes were less regular than those of Group L. Plasma prolactin was significantly lower in Group S than in Group L and there was little evidence for rhythmicity. It is proposed on the basis of the temporal relationship between peaks of prolactin and testis volume in Groups N and L, that prolactin may play a role in the timing of the reproductive cycle in the ram.