In vivo patterns of circulating steroids in adult male rats. I. Variations in testosterone during 24- and 48-hour standard and reverse light/dark cycles.

The in vivo pattern of circulating testosterone (T) was investigated in unrestrained, conscious, individual male rats during 24 and 48 h. Each rat exhibited its own characteristic in vivo diurnal rhythm. When these individual patterns of T were grouped, the mean values also showed a diurnal rhythm. Although T concentrations peaked during the dark and light periods, the most pronounced elevations were observed during the dark periods (2200--2330 h). Lowest T concentrations were noted during the late dark and early light hours (0400--0700 h). The pattern of T was further investigated by extending the experimental period to 48 h. The pattern of T observed during the first 24 h repeated itself on the second day, thus demonstrating the authenticity of this diurnal rhythm. Rats were exposed to a reversed light/dark regimen which resulted in an inversion of the rhythm or circulating T. These data indicate that the pattern of circulating T is not intrinsically regulated.