The purpose of the present study was to elucidate the possible influence of large differences in circannual photoperiodicity on human testicular activity. Serum testosterone was measured once a month in a group of 24 healthy young adult men for a period of 13 months in northern Finland, where the day length is 22 h in mid-summer and 3.5 h in mid-winter. The monthly levels of serum testosterone varied between 11.3 ± 0.93 and 13.0 ± 1.2 nmol/l (SE, n = 24), with no significant differences between individual months. The amplitude of variations in serum testosterone was ± 7% of the annual mean, which was less than in studies reported earlier from central Europe. Our negative findings suggest that the extreme circannual changes in the amount of day-light do not play an obvious role in the regulation of human testicular androgen production.