In humans and rodents the male-to-female ratio of mutation rate (alpha m) has been suggested to be extremely large, so that the process of nucleotide substitution is almost completely male-driven. However, our sequence data from the last intron of the X chromosome-linked (Zfx) and Y chromosome-linked (Zfy) zinc finger protein genes suggest that alpha m is only approximately 2 in rodents with a 95% confidence interval from 1 to 3. Moreover, from published data on oogenesis and spermatogenesis we estimate the male-to-female ratio of the number of germ cell divisions per generation to be approximately 2 in rodents, confirming our estimate of alpha m and suggesting that errors in DNA replication are the primary source of mutation. As the estimated alpha m for rodents is only one-third of our previous estimate of approximately 6 for higher primates, there appear to be generation-time effects--i.e., alpha m decreases with decreasing generation time.