The relationship between partial inhibition of mitochondrial ATP production during the peri-compaction stage and porcine embryonic development was studied. In vitro produced porcine compact morulae were cultured for two days under conditions that should inhibit ATP production via oxidative phosphorylation. The culture conditions included supplementation of the culture medium with sodium azide (NaN3), an oxidative phosphorylation inhibitor; incubation in the presence of 2,4-dinitrophenol (DNP), an uncoupler of oxidative phosphorylation; or incubation under 5% O2 concentration. NaN3 (10-20 microM) increased the average nuclear number found in the resulting blastocysts (P<0.05). The embryos developed in the presence of 100 microM DNP formed blastocysts at a significantly higher incidence than the control embryos (P<0.001); the average nuclear number found in these blastocysts was also higher (P<0.005). When these treatments were applied from the 1-cell stage they proved to be detrimental. Elevations in the frequency of blastocyst formation (P<0.05), and in the average nuclear number per blastocyst (P<0.001) were also measured when compact morulae were incubated in an atmosphere containing 5% vs. 20% O2. NaN3 or DNP did not have negative effects on long term development: the treated embryos were able to form viable conceptuses by day 30 after being transferred into recipients. The data indicate that transient inhibition of mitochondrial ATP production is advantageous for porcine embryonic development in vitro.