The activities of hydroxysteroid dehydrogenases (HSDs) and adenylate cyclase in parthenogenetic mouse blastocysts derived from the ova activated and diploidized by the treatment of ethanol and cytochalasin B were histochemically examined, and were compared with those in control blastocysts developed from fertilized ova. Although the activities of Δ⁵-3β-HSD with DHA and 17α-hydroxypregnenolone as the substrates, 17β-HSD with testosterone, 20α-HSD and 20β-HSD did not differ between parthenogenetic and control blastocysts, the activities of Δ⁵-3β-HSD with pregnenolone as the substrate, 17β-HSD with estradiol-17β and adenylate cyclase were significantly lower in parthenogenetic blastocysts. These results suggest that the metabolism of 17α-hydroxyprogesterone, 20α-hydroxyprogesterone, 20β-hydroxyprogesterone and androgen in parthenogenetic blastocysts is comparable to that in control blastocysts, whereas the metabolism of progesterone, estrogen and cyclic AMP in parthenogenetic blastocysts is low compared with that in control blastocysts. When parthenogenetic blastocysts were treated with hCG, the activities of Δ⁵-3β-HSD with pregnenolone as the substrate and 17β-HSD with estradiol-17β were stimulated, but were still lower than those of control blastocysts. The low potential of development in parthenogenetic embryos was discussed in relation to their steroid metabolism.