The sequence of structural changes in goat hair follicles was investigated using melatonin implants to advance and synchronize spring hair growth. Ten pasture fed cashmere wethers each received a controlled release formulation of 70 mg of melatonin on September 1 1989, and showed plasma melatonin elevated above physiological levels over 14 days post-treatment (914 ± 154 pg/ml [mean ± SEM] on day 14). In ten untreated animals, daytime plasma melatonin was 19.9 ± 4.7 pg/ml. Histological examination of skin biopsies taken over the 14 days from the start of the experiment showed that primary hair follicles of goats with manipulated hormone levels had initiated fiber growth (entered proanagen), whereas primary follicles of untreated goats largely remained in the quiescent phase (telogen). A standardized terminology was used to describe the sequence of events during induced proanagen. Structural reorganization of follicles began in treated animals between days 6 and 12 post-treatment, and emergent fibers grew by day 24. Advancement of spring fiber growth was associated with a suppression of the normal rise in plasma prolactin concentration. Prolactin levels in untreated goats increased from 7.4 ± 1.8 ng/ml on day 1 to 12.8 ± 1.6ng/ml on day 14, but declined in treated goats from 6.3 ± 2.3 ng/ml to 2.2 ± 0.8 ng/ml over the same period. © 1993 Wiley-Liss, Inc.