Summary. Two groups of 6 rams were maintained under constant photoperiodic conditions consisting of short days (8 h light: 16 h dark; Group S) and long days (16 h light; 8 h dark; Group L) from 4 to 20 months of age. Five other rams were reared under a photoperiod representative of that occurring naturally (Group N). Testis size, plasma testosterone and prolactin concentrations were monitored weekly and sexual behaviour tests were carried out at regular intervals. Over the treatment period Groups S and L did not differ in terms of testis growth or plasma testosterone. Both groups had a phase of testis growth and increased testosterone followed by a decline and the temporal patterns for the two groups were equivalent. Sexual behaviour was slower to develop in Group L than in Group S, indicating that photoperiod can affect the development of sexual behaviour irrespective of peripheral plasma testosterone concentrations. Plasma prolactin levels showed a cyclic variation in Group L and were significantly higher overall than in Group S rams. This, together with a trend towards negative correlations between prolactin concentrations and sexual behaviour in Group L, indicates that prolactin may be involved in the effect of photoperiod on sexual behaviour. The presence of a cycle of testicular growth and of hormone concentrations in young animals under constant photoperiod tentatively suggests that these cycles are endogenous. The constant photoperiod did, however, affect the animals because the cycles which occurred in Groups S and L were out of phase with those of Group N by about 4 months.