Melatonin infusion schedule and photoperiodic responses of hamsters in Experiment #2. Panel A illustrates the two melatonin infusion protocols (dashed and solid lines, respectively). Both groups received melatonin signals gradually decreasing in duration from 10 h at week 0 to 7.5 h at week 12 followed by 6 additional weeks of 7.5 h signals. The groups differed in the duration of the melatonin during a 6 week pre-treatment (5 h versus 10 h melatonin, solid symbols and open symbols, respectively). These two regimens were administered to animals entrained to either 15L:9D (circles) or 10L:14D (squares) throughout. Mean (± sem) estimated testis volume (B) and body weights (C) in each of the four resulting conditions. Asterisks indicate time points in which there were significant effects of melatonin duration during pre-treatment (*** p < 0.001; ** p < 0.01; * p < 0.05).