Thirty first and second litters of same parents of the BB/OK rat population were observed for diabetes occurrence up to an age of 200 days. The parents were nondiabetic at conception and pregnancy of their first litter (nd x nd) and either they remained nondiabetic up to an age of 200 days (nd x nd-->nd x nd) or the father (nd x nd-->d x nd) and the mother (nd x nd-->nd x d), respectively, were diabetic at conception of their second litter. There were no significant differences between first and second litters in incidence and age at onset of diabetes in the progeny, so that the genetic constitution of parents and not their metabolic state determines both traits. But, an influence of the maternal age on diabetes incidence of offspring was found. This influence, however, was not related to the metabolic state of mothers. This finding suggests that the age of mothers per se and not the metabolic state of mothers determines the diabetic disposition of the offspring in BB/OK rats.