Abstract When a sperm enters a mature egg of Xenopus laevis both the sperm nucleus and egg pronucleus swell during the first 20 min. Erythrocyte and other nuclei also undergo a similar swelling when injected into Xenopus eggs. We have obtained a similar swelling of hen erythrocyte nuclei in cytoplasm isolated from Xenopus eggs. The swelling occurs in two stages. The first is an immediate small increase in volume similar to that which occurs in media with low concentrations of unbound cations and this can be prevented by adding magnesium ions to the cytoplasm. The second is a slower, irreversible increase to about 40 times the original volume. Neither of these types of swelling occurs in cytoplasm isolated from Xenopus oocytes (as opposed to eggs) but the large irreversible increase in volume can be induced by the addition of heparin. In the light of these experiments we consider the physiological mechanism by which the sperm and egg pronucleus are induced to swell after penetration of the sperm.