The effect of exogenous melatonin on embryo viability in undernourished ewes was investigated. At lambing, 24 ewes were treated (+MEL) or not (-MEL) with a melatonin implant. After 45 days, both groups were fed to provide 1.5 (Control, C) or 0.5 (Low, L) times daily maintenance requirements, so that experimental groups were: C-MEL, C+MEL, L-MEL and L+MEL. Ewes were mated (Day 0) and on Day 5 embryos were recovered and classified according to their developmental stage and morphology. Ovaries were used for in vitro fertilization and uterine horns were processed to study progesterone and oestrogen receptor (PR and ERα) expression by inmunohistochemistry. After 21 days, groups L-MEL and L+MEL had an average weight loss of 10kg (P<0.001). Number of viable embryos per CL from L+MEL (0.50±0.2) was higher than from other groups (P<0.05). Overall, the melatonin effect was particularly evident in undernourished ewes, increasing both viability (L+MEL: 65%; L-MEL: 25%; P<0.05) and pregnancy rates (L+MEL: 66.6%; L-MEL: 16.6%; P<0.05). Neither nutrition and melatonin nor their interaction had a significant effect on the in vitro oocyte development. Melatonin treatment tended to increase the percentage of positive cells to PR in deep glandular epithelium, independently of diet (P=0.09), and the greatest staining intensity of PR was observed in the luminal and superficial glandular epithelia (P<0.0001). In conclusion, melatonin implants at lambing during the breeding season improve the viability of embryos recovered from undernourished ewes, although this effect seems not to be mediated at the oocyte competence level.