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The major cause of lamb mortality in grazing systems at birth is starvation and/or exposure to cold complex, with hypothermia being a primary contributing factor. In precocial species such as sheep, the ability to produce the heat required to prevent hypothermia during the first 12 h after birth relies heavily on the nonshivering thermogenesis, via brown adipose tissue (BAT). Therefore, insufficient BAT deposits, or inability to metabolize BAT, are the major factors contributing to lamb death resulting from starvation/exposure. Circulating concentration of melatonin is associated with proper amount and functionality of BAT. However, there is limited information on the effect of increased melatonin levels, via maternal supplementation, on fetal BAT deposition. The objective was to establish the effect of melatonin implants (M) in single- and twin-bearing ewes, on fetal BAT deposits. Corriedale ewes were synchronized, superovulated, mated to Suffolk rams, and managed under commercial grazing conditions. Single (S)- and twin (T)-bearing ewes received 0 (M0), 1 (M1), or 2 (M2) commercial 18-mg melatonin implants (Regulin) at 100 d of gestation (n = 8 per group). Ewes were euthanized at d 140 of gestation, and total fetal perirenal fat (BAT) was excised and weighed. The effect of litter size (S vs. T), number of implants (M0, M1, or M2), and their interaction on fetal biometrics were analyzed using ANOVA. A rank × treatment interaction (P = 0.002) was observed for total BAT, where SM2 fetuses tended to have 18% more BAT compared with SM0 (P = 0.1) and SM1 fetuses (P = 0.09; 22.8 ± 1.6 vs. 19.3 ± 1.5 or 19.2 ± 1.5 g), whereas TM1 fetuses had approximately 35% more BAT compared with TM0 (P = 0.0002) and TM2 fetuses (P = 0.0003; 22.9 ± 1.1 vs. 17.1 ± 1.0 or 16.9 ± 1.1 g). Single and twin M2 fetuses tended to be 5 to 8% heavier compared with single and twin M0 and M1 fetuses (4.1 ± 0.1, 3.8 ± 0.1, and 3.9 ± 0.1 kg, respectively; P = 0.09). In addition, M2 fetuses, compared with M0 and M1 fetuses, showed greater fetal thorax diameter (34.6 ± 0.4, 33.8 ± 0.3, and 33.9 ± 0.3 cm, respectively; P = 0.047) and presented a trend for increased crown-rump length (44.1 ± 0.5, 43.0 ± 0.4, and 42.6 ± 0.5 cm, respectively; P = 0.056). These results indicate that maternal melatonin implants from d 100 of gestation increases BAT deposition, especially in twin fetuses, and may increase BW. Both effects may have important implications for newborn lamb survival under commercial grazing conditions. Funded by CONICYT project number 11150998. [ABSTRACT FROM AUTHOR]