Pineal melatonin content in photoperiodically responsive and non-responsive phenotypes of deer mice.

1. The pineal melatonin content of photoperiod responsive and photoperiod non-responsive male deer mice was assessed. 2. Pineal melatonin content did not differ between short-day mice with regressed testes or those that maintained their reproductive systems. 3. These data suggest that differential phasing of melatonin release or post-pineal mechanisms are responsible for the differential responsiveness to day length among deer mice.