Male red deer calves, whose mothers had been kept for the last 14 weeks of gestation in long days (18 h light:6 h dark) (group L, n = 7) or short days (6 h light:18 h dark) (group S, n = 5), were kept in constant intermediate daylength (12 h light:12 h dark) from birth to 75 weeks of age. Both groups showed the same live-weight gain. Mean plasma LH concentrations were higher in group L than in group S from birth to 20 weeks of age (averaging 1.55 versus 0.48 ng ml-1, P < 0.001), from 21 to 45 weeks (1.65 versus 1.32 ng ml-1, P < 0.05) and from 46 to 50 weeks (1.84 versus 1.27 ng ml-1, P < 0.001); thereafter, there was no significant difference between the groups (1.81 ng ml-1). Mean concentration of plasma testosterone was relatively low from birth to 30 weeks (averaging 0.38 and 0.27 ng ml-1 (P < 0.05) in groups L and S, respectively), but thereafter increased to a maximum which was greater (2.78 versus 1.46 ng ml-1, P < 0.01), and occurred earlier (47 versus 68 weeks of age, P < 0.001) and at lower body weight (82 versus 96 kg, P < 0.01) in group L compared with group S. Growth of antlers started in both groups at 25 weeks, but they hardened earlier in group L than in group S (42 versus 47 weeks of age, P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)