Background The influence of melatonin (MT) on the onset of puberty has attracted many researchers leading to many contradictory claims. Although some researchers believe that exogenous MT delays the onset of puberty in female juvenile rats, others dispute this finding. We explored the effects of exogenous MT, administered for different periods of time, on very young pups. Methods Groups of 5‐ and 10‐day‐old rat pups were given daily 100 μg of MT subcutaneously for 10, 15, and 20 days at 1500 hours. On the vaginal opening day (VOD), rats were sacrificed for collection of ovary and thymus. Besides measuring ovarian and thymus weights, Graafian follicular ratio and thymic corticomedullary ratio were computed. Results MT caused an advancement of VOD in 5‐day‐old pups but delayed it in 10‐day‐old ones. Other parameters studied revealed an exciting correlation with this reversed trend in VOD. In the 5‐day‐old groups, the ovaries and thymuses showed an increase in weights, but the 10‐day‐old groups recorded a fall. Follicular and thymic corticomedullary ratios also displayed corresponding increases and decreases. Conclusions MT administered to younger pups (5 day old) causes an earlier VOD probably due to an ovarian stromal inhibition. In slightly older pups (10 days old), where most secretory mechanisms are progressing from an immature to an adult type, perhaps exogenous MT depresses the developing and sensitive thymus‐hypothalamus‐pituitary‐gonadal axis (THPGA) and causes a delay in VOD. © 1996 Wiley‐Liss, Inc.

MT administered to younger pups (5 day old) causes an earlier VOD probably due to an ovarian stromal inhibition. In slightly older pups (10 days old), where most secretory mechanisms are progressing from an immature to an adult type, perhaps exogenous MT depresses the developing and sensitive thymus-hypothalamus-pituitary-gonadal axis (THPGA) and causes a delay in VOD.