Twelve adult Limousine rams (five pinealectomized, four sham operated, and three control) were housed under an artificial lighting regime of alternating periods of long (16L:8D) and short (8L:16D) days for 18 months, and long‐term variations in kemp follicle growth were recorded along with measurements of the plasma prolactin concentrations. In control 2nd sham‐operated rams, both parameters varied in relation to imposed lighting regime. Moult and growth of the kemp follicles occurred during each short‐day period at a time when the concentrations of prolactin were low and vice versa. Variations in the pinealectomized rams were different from those in the control or sham‐operated ones. There was no synchronous periodic kemp activity: some follicles remained active at all times, and the seasonal pattern of plasma prolactin levels disappeared. The relationship between the kemp follicle cycle and the pattern of plasma prolactin concentrations and its control by the pineal gland are di