Primary spermatocytes originating from prepubertal mouse testes were electrofused to metaphase II (MII)-stage oocytes, enucleated either by the conventional micromanipulation method or by chemical treatment with etoposide and cycloheximide. These experiments were followed by assessment of morphological changes in transferred nuclei using light microscopy, by chromosomal analyses and by screening of hybrids for the presence or absence of DNA synthesis using anti-bromodeoxyuridine antibody and immunofluorescence staining of the hybrids. The results show differences between the two types of ooplasts in susceptibility to activation stimuli. However, when activated, both types of ooplasts gave rise to hybrids of similar morphology. From 35.3% to 63% of activated hybrids originating from chemically or microsurgically enucleated oocytes, respectively, contained one large pronucleus in cytoplasm, 62% or 31.6% hybrids from those two groups, respectively, possessed two smaller pronuclei and a few contained three or four pronuclei. No DNA synthesis was detected in any hybrid containing one or more pronuclei. The chromosome spreads of hybrids with premature chromosome condensation (PCC) morphology (before activation) show that most of the hybrids had a diploid (2n) number of chromosomes. The nature and regularity of the cell division cycle in the hybrids are discussed.