The objective of this study was to test the hypothesis that a combination of induced estrus (IE) and melatonin (MEL) would increase reproductive performance in breeds characterized by late maturity more than either IE or MEL alone. Spring-born Columbia (C; n = 161; 188 to 222 d range of age at breeding; 49 to 80 kg range of BW at breeding) and Targhee (T; n = 166; 183 to 210 d range of age at breeding; 40 to 68 kg range of BW at breeding) ewe lambs were stratified randomly by breed to treatments: ambient controls (AC); IE; MEL; and MEL+IE. Melatonin (18 mg of Regulin) was implanted on September 7. Estrus induction included medroxyprogesterone acetate (MAP) pessaries inserted for 12 d and pregnant mare's serum gonadotropin (PMSG; 500 IU i.m.) at time of MAP removal. Ewe lambs were placed into two pens with five rams per pen by breed. Targhee ewes lambed at an increased rate with IE vs no IE (84.5 vs 67.9%; P < .01) and tended to lamb at an increased rate with MEL vs no MEL (82.5 vs 70.6%; P = .07). The 92.7% lambing rate observed in the Targhee ewe lambs for the combined treatment (MEL+IE) was slightly higher than an additive effect but the interaction between IE and MEL was not significant (P = .24). By contrast, none of the treatments altered (P = .5) lambing rate, number of lambs born, or number born alive for Columbia ewe lambs.(ABSTRACT TRUNCATED AT 250 WORDS)