The Brattleboro diabetes insipidus mutant is incapable of synthesizing vasopressin, but is activated in its production and release of oxytocin. In the homozygous female mutant, there were abnormally short and long oestrous cycles and reduced conception rates after mating with mutant males, a shortened gestation length, and a shorter time-course of labour. A better conception rate when mutant females were mated with normal Brattleboro males indicates the involvement of a male factor in the subfertility. The reduced litter size in diabetes insipidus rats appeared neither dependent of the male's genotype, nor due to a decreased viability of diabetes insipidus fetuses.