The effect of melatonin treatment on fur maturation period and hair follicle cycle in 4-month-old male and female chinchillas, kept under conditions of natural photoperiod, was studied. The animals were treated with continuous-release implants of melatonin (18 mg, group M, no. = 56), while no treatment was given to the control (group C, no. = 69). The effect on hair follicle activity cycle under conditions of normal hair growth (no. = 8) and after defleecing (no. = 8) and on number of hair fibres per follicle bundle (no. = 27) were measured by taking skin samples for histological examination once a month. The age at fur priming was reduced by 31 days (P < 0001) in the melatonin-treated chinchillas. When moulting began during the short-day period, the fur reached maturity 13 days (P < 005) sooner than when the moult began during the long-day period. Melatonin administration proved to be effective in these two periods. Melatonin treatment led to hair follicle activity decreasing at a faster rate, and the differences between treated and control groups from day 60 were significant (P < 005). The primary and secondary hair follicles reached the telogen phase 30 days earlier. Due to melatonin administration more fibres per follicle bundle were observed on the matured pelt (P < 005). At 4 months of age, following the removal of hair by defleecing, the growth of the first adult hair was studied without the presence of young-age hairs. No influence of defleecing as opposed to normal hair development on age at fur maturation or on the number of fibres per hair follicle bundle was observed. Within groups M and C the number of fibres per hair follicle bundle proved to be independent of age at first fur priming. No substantial differences between the groups were observed with respect to either body weight or pelt length at the time of pelting. According to these results, melatonin administration is effective in shortening the coat maturation period in growing chinchillas and causing corresponding changes in the hair follicle cycle.