Summary. Variations in the incidence of pregnancy-block (Bruce effect) dependent on the genotype of the male were recorded in female mice belonging to two inbred strains. CBA females invariably responded with a high rate of return of oestrus irrespective of the blocking agent, KE and CBA males, or spayed and masculinized females belonging to these two strains. By contrast, KE females only react strongly to CBA males and masculinized CBA females. Their response to males of their own strain is insignificant