Non-surgically collected day 7 bovine embryos were microsurgically cleaved and reinserted into surrogate zonae pellucidae. The demi-embryos were fixed immediately after splitting and at various intervals of in vitro incubation at 35 degrees C in modified Dulbecco's medium. At the light-microscopical level, the demi-embryos were found to have restored their prebisection morphology within 30 min after splitting. The electron microscopy confirmed these findings, by the presence of mitosis, cell polarization, neoformation and reaggregation of mitochondria, increased pinocytosis and cell-to-cell contact between trophoblastic and/or inner cell mass cells. The present observations therefore suggest that day 7 bovine demi-embryos intended for transfer should be cultured for 30 min before being morphologically evaluated.