Fig. 1.

Serum levels of melatonin at the corresponding hours of acrophase and nadir in young and old ringdoves, either non-treated or after 3 days of treatment 1 h before lights out with 0.25 mg/kg b.w. and 2.5 mg/kg body weight (b.w.) melatonin, or after 3 days of treatment 1 h after lights on with 300 mg/kg b.w. tryptophan in old ringdoves. Each value represents the mean ± standard deviation of ten determinations performed in duplicate. Lower case letters indicate P < 0.05: a vs values obtained in the diurnal period, b vs values obtained in the corresponding non-treated groups, c vs values obtained in young animals