The frequency of urogenital abnormalities in adult ACI rats was 18.8%. These abnormalities consisted of a spectrum of defects ranging from focal aplasia of the ureter to the complete absence of the ureter, kidney, and tubular genital tract on the affected side. When ACI rats with normal urogenital tracts tracts were crossed with F344 rats the frequency of the defects decreased to a constant level in the F1, F2, and F3 generations. There was no clearly significant decrease in the frequency of the defects in the backcross offspring of (ACI times F344)F1 times ACI matings, but there was a significant and dramatic decrease in the backcross to F344 rats. Attempts to select against these defects by brother-sister matings among normal ACI rats were not successful: 5 generations of inbreeding did not alter the frequency of the urogenital abnormalities. The frequency of the abnormalities was the same in the offspring of affected parents as in the offspring of normal parents, indicating homozygosity of the factors responsible for the defects. The results suggest that the transmission of these defects is polygenic.