With the present study, we aimed to investigate the effects of melatonin on the initiation of breeding season in the female cats. For this purpose, 12 female cats and 2 tom-cats (male cats) (vasectomized) at age of 2-6 years old were used. The female cats were randomly divided into control (group 1, n = 6) and melatonin-treated groups (group 2, n = 6). Sham (placebo) and melatonin implants (containing 18 mg of melatonin) were placed subcutaneously into the females during the late anoestrous in groups 1 and 2, respectively. Sexual behaviors, vaginal smears and serum progesterone levels for the animals were recorded. The effect of the daylight (varying from 9.5-12.5 h, December through April) on the cyclic activity were also assessed during the study period. The animals were sexually silent during the anoestrus period. Upon the commencement of breeding season, while the female cats in group I showed estrus signs, none but one of the female cats in group 2 possessed the indications of the estrus. Moreover, the findings of cytology were not statistically different in both groups during the study period (p >= 0.05). Although, the progesterone levels in breeding season were higher in group 1 (p <= 0.05) than in un-breeding season, no such difference was present in group 2. The progesterone levels in breeding season were markedly elevated (p <= 0.01) in group I with comparison to group 2. In the latter group, melatonin effectively postponed cyclic activity of the females. The present results suggest that the administration of exogenous melatonin might prolong un-breeding season, presumably via the suppression of hypothalamic centers. Besides, we found that exposure to daylight of 10.5 or more hours per day were also, sufficient for the onset of cyclic activity in the female cats.