Diel plasma testosterone rhythms in male Microtus montanus, the montane vole, under long and short photoperiods.

Diel plasma testosterone rhythms were examined in adult male Microtus montanus, a seasonal breeder which displays nocturnal locomotor activity under long photoperiod and diurnal activity under short photoperiod. Under 16 hr light:8 hr dark (16L:8D), plasma testosterone levels peak during the light at 11.8 ng/ml. Under 8L:16D, plasma testosterone peaks during the dark at 5 ng/ml.