AbstractRed deer calves were reared from birth to 16 months in either constant intermediate 12L: 12D daylength (ID) or in natural photoperiod (NP) (four males and four females per group) to investigate effects on the somatotropic and reproductive axes, and to compare responses between the seres. Measurements, starting from 3 months (September), were made each week of live weight (LW), voluntary food intake (VFI), plasma prolactin, plasma insulin-like growth factor-1 (IGF-1), and plasma progesterone (females), and every 2 months of pulsatile LH secretion, plasma testosterone (males), responses to exogenous GnRH, and antler development (males).Both sexes in ID compared with NP had significantly higher LW gain (males, P < 0·001; females, P < 0·01) and VFI (P < 0·001) between winter solstice (WS) and spring equinox (SE), and VFI between SE and summer solstice (SS) (males, P < 0·05; females P < 0·01). Both sexes had significantly lower plasma prolactin concentrations in ID than in NP (males, P < 0·05; females, P < 0·01) between SE and SS. However, plasma IGF-1 was only significantly altered in males, being significantly higher in ID than NP between WS and SE (P<0·01). ID and NP females showed no significant differences in pulsatile LH secretion nor in the timing of pubertal ovulation. However, ID compared with NP males at 10 months (just after SE) had higher LH and testosterone pulse frequencies (P < 0·01), and at 12 months (just before SS) had higher mean testosterone concentrations (P < 0·01) and testosterone response to GnRH challenge (P<0·001). ID antlers hardened earlier at 11 months than NP antlers at 14 months (P < 0·001).Thus ID compared with NP in both sexes prevented the winter reduction in growth and appetite and the summer elevation in prolactin secretion, and in males, but not in females, stimulated higher IGF-1 secretion in winter, an earlier increase in LH pulse frequency, and an earlier increase in gonadal steroid production. This study has therefore revealed some intriguing similarities and contrasts in the responses to photoperiod shown by young male and female deer.