Crossbred beef heifers (n = 144) were assigned at weaning (187 ± 14 d of age) by body weight (225 ± 23 kg) and sire breed (British/Continental) to one of two photoperiod treatments from 21 Decem ber 1998 (0 wk) until 10 May 1999 (20 wk): natural photoperiod (NP) that gradually increased from 8.1 h (0 wk) to 15.2 h (20 wk) and, extended photoperiod (EP) that consisted of natural + supplemental light (400 lx, 1 m above ground) to extend photoperiod to 16 h. Rations were formulated for two-steps of body weight gain (0.6 and 1.2 kg d-1) to achieve 60% of mature weight at 18 wk. Visual observations of estrus behavior were made twice daily and confirmed by serum progesterone. Body weight, backfat and serum prolactin data were determined for each 4-wk period. Ambient temperatures averaged -12.2 ± 6°C in winter (0 to 12 wk) and 4.2 ± 5°C in spring (12 to 20 wk). Gain in body weight was greater (P < 0.05) and backfat lower (P < 0.05) for EP than NP treatments from -2 to 6 wk and only 1% of heifers had attained puberty during this period. However, as yearlings at similar (P > 0.05) body weight and backfat, more (P < 0.05) EP than NP heifers had attained puberty (84.7% vs. 69.4%). Prolactin was greater (P < 0.05) for EP than NP treatments from 2 to 6 wk (10.3 vs 5.5 ± 1.2 ng mL-1). Management of photoperiod influences attainment of puberty and prolactin secretion in beef heifers housed in an outdoor environment. Key words: Photoperiod, puberty, estrus, beef heifers, prolactin