The aim of the study was to analyse the annual changes in the concentration of free triiodothyronine, progesterone and testosterone in the serum of prepubertal red deer males and females. Blood was collected monthly from 8 stags and 8 hinds for a period of one year, beginning in November. Hormone concentrations were determined using an immunofluorescence method (TR-FIA). Monitoring of the duration of the prepubertal period, based on the progesterone concentrations in females and testosterone concentrations in the males, suggested that the prepubertal period ended in September in both sexes. The experiment revealed the annual rhythm in free triiodothyronine (FT3) secretion, with significantly higher concentrations in the winter-spring period in comparison with summer-autumn in both, males (P < 0.01) and females (P < 0.05). The results showed that the season considerably influenced the intensity of FT3 secretion in prepubertal red deer and that the secretion dynamics in both sexes were similar, however, the concentrations in the males reached higher levels than in the females in each season and across the whole experiment