Adult female mice, regardless of the stage of the oestrous cycle, were superovulated with PMSG and hCG. Ovulated oocytes were recovered 20-22 h after hCG and fertilized ova 72-74 h after hCG. Compared with the controls, the gonadotrophin treatment increased the mating rate of the females, and the incidence of abnormal ova. Regardless of the site of gonadotrophin injections, the numbers of ova were equal, but the proportion of abnormal eggs in mice injected intraperitoneally was significantly higher than in mice injected subcutaneously.