Abstract One-cell mouse embryos were cultured in several concentrations of pyruvate and lactate. Maximum development to blastocysts occurred when one-cell ova were cultured in media containing 0.25 mM pyruvate during the first cleavage division and 30.00 mM lactate plus 0.25 mM pyruvate after the first cleavage division. The unusual sensitivity of one-cell ova to both the kind and quantity of energy source was not evident on day 2 of development; normal appearing two-cell ova were formed under extreme conditions of up to 100.00 mM pyruvate and 90.00 mM lactate. The data demonstrate that the successful development of one-cell ova in vitro depends on satisfying separate requirements for the first cleavage division versus development after the first cleavage division. The formation of morphologically normal two-cell ova cannot be used as the sole criterion for satisfying the requirements of the first cleavage divison.