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Diurnal variations in testosterone in plasma were studied in two inbred strains of mice, BALB/cBy and C57BL/6By. Blood was taken every 4 h over 24 h from male mice at 70 days of age using a lighting regimen of 12 h light to 12 h darkness (lights on 07.00–19.00 h). Values of testosterone in plasma were transformed to log(testosterone in ng/ml) to reduce inequality of variance between groups. In both strains, the distribution of pooled values over all times of day was bimodal, and bimodality was present at most times of day. Circadian variation was evaluated by dividing the transformed values into high and low modes at each time of day and testing for significant variation in the number of animals in each mode over time using the chi-squared test. Significant circadian variation was found in the BALB/cBy strain of mice but not in the C57BL/6By strain. The highest number of high mode cases for BALB/cBy mice was at 22.00 h and the lowest number of high mode cases was at 10.00 h. The log transformation and bimodality of these values are presented as biological expressions of blood levels of testosterone and of tissue responses to these levels in the male mouse. The strain difference in circadian variation may be related to reported circadian changes in behaviour and to possible genetic effects on sensitivity to environmental change or capacity to express circadian rhythms.

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