Two-cell rat embryos were exposed to one or a train of two electric pulses with different field strengths and durations. It was difficult to induce blastomere fusion with a field strength of 1.0 kV/cm of 50μs duration: only 1 (7%) of 15 treated embryos fused after either one or a train of two pulses. The rate of fusion reached a maximum (80-100%) with 1.5-2.0 kV/cm and with 100-200μs duration. While the fusion rates were improved when the embryos were treated with a train of two pulses of 1.0 kV/cm for 100-200 μs, little effect was observed with 1.5 and 2.0 kV/cm. With 1.0 and 1.5 kV/cm, the fusion times were shortened when the embryos were exposed to a train of two pulses of 50 to 200 μs duration; no marked shortening of the time by the increased number of pulses was observed with 2.0 kV/cm. When fused embryos were transferred into pseudopregnant females and recovered 3 days later, 15 (83%), 12 (80%) and 9 (64%) of recovered embryos, which were originally submitted to 1.0, 1.5 and 2.0 kV/cm of field strengths each for 150 μs, respectively, had developed to the morula-blastocyst stage. There were no significant differences in the rates of development among these different field strengths.