The object of the study was to assess the lability of testosterone levels in plasma of normal human males over a long period of time and to search for periodicities in changing levels. Blood samples obtained from 20 healthy young men every second day for 2 months were assayed for total testosterone concentration by radioligand saturation analysis with late-pregnancy plasma. The flucturations of plasma testosterone levels over the total time span were substantial for most individuals; the coefficients of variation ranged from 14 to 42% (median 21%). The presence of periodic functions in these fluctuations was tested by 4 different, relatively independent methods. Close agreement among at least 3 analytic methods was found for 12 out of the 20 subjects. These 12 subjects had cycles of plasma testosterone levels with periods ranging between 8-30 days, with a cluster of periods around 20-22 days. The majority of such cycles were significant at least at the 5% level. The mean amplitudes of these cycles ranged from 9 to 28% of the subjects' mean testosterone levels (average 17%).