Micromanipulation of human oocytes and embryos has provided new opportunities for both the treatment of infertility and the preimplantation diagnosis of genetic disease. It is important to determine whether manipulated embryos develop normally in vitro, as an indication of their suitability for transfer. However, at present there is little information on the development of non-manipulated embryos in vitro for comparison. We have therefore monitored morphological changes and human choriomc gonadotrophin (HCG) secre tion in 36 non-manipulated human embryos, including 26 blastocysts and 10 cavitating morulae, daily from day3 to day 14 of culture. Hatching was observed in 10 (38.5%) blastocysts and five of these adhered to the culture dish and appeared viable until day 14. The secretion of HCG was first detected on day 8, peaked at day 10 (51.11±8.7 mIU/ml) and then declined but was still detectable in four blastocysts on day 14. There was no overall difference in HCG secretion by hatched blastocysts and those which remained within the zona. However, those hatched blastocysts which showed adherence had significantly increased (P <0.05) HCG secre tion. For individual blastocysts, the pattern of HCG secre tion correlated well with the assessment of morphology. These data provide the basis for comparative studies of morphological changes and HCG secretion in manipulated embryos.