Twenty-four castrated male lambs initially maintained on a photoperiod of 12 h light: 12 h dark were allocated to a factorial experiment with two daylengths 8 h L: 16 h D or 16 h L: 8 h D) and two levels of feeding (restricted or ad libitum). Blood samples were taken every 4 h for 24 h during the introductory period and after 24, 51 and 79 days of treatment. There were highly significant positive effects of daylength and level of feeding on serum prolactin: mean concentrations increased from a mean of 38 ± 1 ng/ml during the introductory period until at day 79 they were: 8L:16D (restricted diet), 81 ng/ml; (food ad libitum), 167 ng/ml; 16L:8D (restricted diet), 262 ng/ml; (food ad libitum), 262 ng/ml (s.e. of treatment mean ± 4). Long daylength and feeding ad libitum also significantly increased growth rate.