Breeding experiments demonstrated that the average age at onset of fertility of male Hre (restricted color) rats was 71 ± 9 (SD) days, while normal color (N) males were fertile by 65 ± 6 (SD) days of age. Two of the ten Hre males studied were never fertile. Length of continuous fertility in Hre males was highly variable (range = 0-122 days) and averaged 48 ± 49 (SD) days.