The level of protein degradability in ruminant diets, notably in the peri-ovulatory period and during the early weeks of gestation, can significantly affect conceptus survival and may even interfere with normal control of foetal growth. For example, high levels of rumen degradable protein (RDP) increase embryonic mortality in superovulated sheep but, paradoxically, also have been linked with foetal and neonatal oversize in a small number of cases (Bishonga et al, 1994; Animal Production 58: 447). The present study examined whether there was any evidence of altered development at the early pre-implantation stage in ova derived from ewes offered high levels of dietary RDP in the form of urea, and also investigated the possibility of systemic and local micro-environmental effects in vivo.