Conception rates and litter sizes in ewes are often higher during the peak of the breeding season than at the beginning or end. How much of this is due to seasonal differences in ovulation rate (Montgomery et al 1988) and ram fertility (Colas, 1983) is uncertain. Using laparoscopic intrauterine insemination which virtually eliminates fertilization failure, Aitken et al (1990) obtained estimates of embryo survival for Greyface ewes induced to ovulate in mid-June (mid anoestrus) that were 25, 19 and 13% below the expected values for 2, 3 and 4 ovulations respectively. While this implies that embryos transferred to recipient ewes at an induced oestrus during seasonal anoestrus may have sub-optimal survival rates, it does not identify whether the cause lies solely with the embryo or with the uterine environment or with both. The present study employs reciprocal embryo transfer in conjunction with melatonin treatment to identify the influence of seasonality on the inherent viability of the embryos and the ability of the uterine environment to sustain pregnancy.