Determination of oestrous condition in female mice is dependent upon time of day

Mice were maintained in rooms with constant ambient conditions and a 12 h light:12 h dark daily light regimen with the lights on starting at 06:00 h. In Exp. I, detection of puberty, as measured by first vaginal oestrus, was recorded at significantly earlier ages when vaginal smears were taken at 04:00 or 08:00 h than at other times of the day. In Exp. II, adult females were detected as being in oestrus more frequently when vaginal smears were taken within 6 h before or after the onset of the light part of the cycle each day than at other times.