Prenatal photoperiod influences postnatal prolactin secretion and the timing of reproductive development in male red deer reared from birth in a constant equatorial photoperiod (12:12 light:dark). The present trial investigated whether a similar phenomenon occurs in female red deer. Female deer whose mothers had been exposed for the last 14 weeks of gestation to long (group L, 18:6 light:dark) or short day length (group S, 6:18 light:dark) were kept from birth in constant equatorial day length with food available ad libitum. Both groups showed similar live-weight gain to 90-100 weeks of age. Blood samples taken once or twice weekly were analyzed for progesterone and prolactin. Progesterone concentrations indicated that there was no difference between the groups in the timing of the first incidence of ovarian (luteal) activity, which occurred at a normal or late age for natural puberty (67 weeks or older). Only one individual per group exhibited normal repeated luteal cyclicity since there was a high incidence of irregular or abnormal luteal function. Plasma prolactin concentrations at birth were higher in group L than group S (P < 0.001). Thereafter, although the mean and peak values did not differ significantly between the groups, there was a significant difference in the pattern of secretion; deer in group L showed significant clustering of prolactin peaks (P < 0.01) at a mean age of 48 weeks, whereas deer in group S showed a random distribution of peaks. Therefore, for female red deer raised in constant equatorial photoperiod, prenatal long day lengths did not advance timing of puberty.(ABSTRACT TRUNCATED AT 250 WORDS)