The aim of this study was to assess the effects of melatonin on the in vitro maturation and fertilization of sheep oocytes, and on the in vitro culture of the embryos. Oocytes from sheep ovaries collected at the slaughterhouse were divided into four groups, two of which were treated with either 10–5 M (M5) or 10–6 M (M6) melatonin, while the other two groups served as untreated controls (C5 and C6). After in vitro fertilization with fresh ram semen, the embryos produced in each group were divided into two sets, one cultured with melatonin (M5M, C5M, M6M and C6M), and the other without melatonin (M5C, C5C, M6C, and C6C). A melatonin concentration of 10–6 M increased maturation rate (82.5% vs. 73.7% for M6 and C6, respectively; P < 0.05) and tended to improve cleavage rate 36 hours after in vitro fertilization (79.4% vs. 72.6% for M6 and C6, respectively, P = 0.08). A higher melatonin concentration (10–5 M) did not have significant effects on those parameters. Blastocyst rates on day 8 did not differ significantly among groups. Additional key words: in vitro fertilization, in vitro maturation.