This study investigated the effect of melatonin treatment of ewe lambs on LH pulsatility in an attempt to examine the mechanism whereby melatonin advances the onset of puberty. Six ewe lambs were given intravaginal melatonin implants at 12.8 weeks of age. Another six lambs received empty implants. All lambs were serially blood sampled every 15 minutes for six hours on several occasions prior to the onset of puberty. One week after implantation LH pulse frequency and mean LH levels were higher in treated lambs than the control lambs (pulse frequency 0.13/h vs 0.03/h; mean LH levels 2.0 ± 0.2 ng/ml vs 1.3 ± 0.1 ng/ml; p < 0.05). Melatonin treatment failed to alter pulse frequency after the initial increase. Puberty was advanced by 3 weeks in the treated group. In the second experiment six lambs received melatonin implants at 13 weeks of age and another six lambs served as control. In this experiment blood samples were taken intensively during the first few weeks after treatment. Results of this study show that mean plasma LH levels and LH pulse frequency were again higher during the first week after implantation. This transient increase in LH release may be part of the mechanism initiating the eventual advancement of puberty although the significance of this increase is questionable. In both experiments the LH response to estradiol injection was monitored at various times after treatment, but no effects of melatonin were found, although the magnitude of the response increased with age.