Sixty 6- to 10-week-old male Chinese hamsters were assigned to two groups. In one, the animals were untreated and in the other, they were treated with 0.02 g caffeine/100 ml water orally for 60 days. The sex ratio of the resulting litters showed a significant (P smaller than 0.025) skewing towards females (61.4%) when compared to that of the controls (49.2%).