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On the day of diestrus, female mice, syrian hamsters and rats, showing regular 4-day estrous cycles, were injected with hCG or PMSG and were inspected for the presence of ovulation the following day. The dose level expected to cause an effect in 50% of the animals (ED50) was calculated using the Van der Waerden method. When hCG was injected into i. v., i. p. and s. c., the ED50 values per animal and per body weight (kg) in parenthesis were as follows; 0.2 (7.7), 0.3 (11.5) and 0.7 (26.9) I. U. for mice, 1.0 (9.5), 1.8 (17.1) and 2.6 (24.8) I. U. for syrian hamsters and 1.3 (4.6), 3.5 (12.3) and 7.5 (26.3) I. U. for rats, respectively. In PMSG study, the ED50 values per animal and per body weight (kg) in parenthesis were as follows: i. v., 0.8 (30.8); i. p., 2.0 (76.9); s. c., 2.8 (107.7) I. U. for mice, i. v., 3.6 (34.3); i. p., 8.0 (76.2); s. c., 13.2 (125.7) I. U. for syrian hamsters and i. v., 6.0 (76.8); i. p., 20.8 (73.0); s. c., 76.8 (269.5) I. U. for rats, respectively. From these results, the intravenous ED50 value was lower than other routes in three rodents with hCG or PMSG. In all injection routes, the ED50 value for mouse was lower than others. However, there were not significant differences in the ED50 values per body weight (kg) among three rodents. In particular, subcutaneous ED50 of hCG and intraperitoneal ED50 of PMSG were almost same values among three rodents, respectively. Given that the ED50 value per body weight (kg) in one of three rodents is determined, its value may be possible to be extrapolated to remaining two rodents.