The tolerance of the oocyte plasma membrane (oolemma) to electrical pulses (TEP) was investigated for oocytes, zygotes, and embryos at the early and late 2-cell stage. The oocyte survival rate after two electrical pulses (1.4 kV/cm for 640 microns each) was used as an indicator of the TEP of the oolemma. Survival rate of mid-pronuclear zygotes (94.3% +/- 2.3%) was significantly (p < 0.05) higher than that of recently ovulated (2.1% +/- 1.9%) and in vivo aged (25.1% +/- 2.6%) oocytes; survival rate of in vivo aged oocytes was also significantly higher than that of recently ovulated oocytes. Soon after fertilisation, the survival rate of the oocytes markedly increased, up to 94% at the mid-pronuclear stage. Survival rate dropped thereafter. These results suggest that the characteristics of the oocyte plasma membrane (oolemma) change after fertilisation.