Exposure of New Zealand Wiltshire sheep to long days, following 24 weeks of short days, caused a synchronised out-of-season wool follicle growth cycle. Skin biopsies were collected at intervals between 3 and 30 days and follicles were examined by light microscopy in both transverse and longitudinal section to describe the regressive (catagen), resting (telogen) and regenerative (proanagen) stages of the induced growth cycle. Follicles were generally in the growing phase (anagen) during short day treatment but by day 20 after exposure to long day photoperiod, 16% of follicles were in late catagen. By day 52, all follicles were in various stages of catagen, telogen and proanagen. The progression through the cycle occurred more slowly, but was morphologically similar to follicle growth cycles reported in rodents and goats, induced by plucking or melatonin, respectively. Follicles in early catagen were rarely observed, possibly reflecting the brevity of this phase of the cycle. Late catagen follicles were distinguished by the presence of a brush end and an inner root sheath, the latter disappearing as follicles entered telogen. Immunocytochemistry of proliferating cell nuclear antigen provided evidence that mitotic activity in the follicle bulb ceased completely during the brief telogen phase. The simultaneous absence of type I intermediate filament keratin mRNA indicated that keratinocyte differentiation had also been interrupted. Cell proliferation was re-established in early proanagen prior to observable changes in the follicle microanatomy. The relatively synchronised follicle growth cycle induced by photoperiod manipulation represents a potentially useful model for the study of changes in follicle ultra-structure and the endocrine and biochemical regulation of seasonal hair growth patterns.