Experimental protocol and photoperiodic responses of hamsters in Experiment #3. Mean (± sem) estimated testis volume (ETV) (B, C) and body weight (D, E) of hamsters during four weeks of exposure to implants of melatonin in beeswax or beeswax alone (B, D) and during 14 weeks of exposure to 13L:11D (C, E). In (A), panels illustrate the daylength to which the circadian system was presumably entrained. The shaded area indicates where melatonin capsules obscured the endogenously generated melatonin signal associated with the ambient daylength. Hamsters exposed to long (LD) or short (SD) daylengths during Weeks 2–4 (15L:9D versus 10L:14D) were maintained in those photoperiods (SD/SD and LD/LD) or transferred to the opposite condition from Weeks 4–7 or Weeks 4–8 (SD/LD and LD/SD; see text for details). All hamsters were in 13L:11D from Week 8 onwards.