The metabolic rate of 14C-phenazepam (14 mg/kg) in male C57BL/6 and BALB/c mice and F1 hybrids (BALB/c X C57BL/6) and the anticonvulsant effect of the tranquilizer in hybrid mice were studied. It was demonstrated that oxidation of the phenazepam molecule in the third position in BALB/c mice and in F1 hybrids proceeded at a higher rate than in C57BL/6 mice. The sensitivity to the anticonvulsant effect of phenazepam in C57BL/6 animals was greater than in BALB/c mice and F1 hybrids.