Mice homozygous for rho-25h, a mutation at the pink-eye locus, are small, nervous and male sterile, with abnormal sperm morphology: females have very low fecundity. Evidence is presented that in both sexes the pars nervosa of the rho-25h/rho-25h pituitary contains degenerating nerve axons, that the hypothalamus has a reduced binding capacity for oestradiol-17 beta and that thyroid iodine binding is reduced. No changes were observed in the anterior pituitary. In rho-25h/rho-25h females, the uterus is threas-like and unresponsive to oestradiol, and the ovaries are small with many abnormal follicles. Possible modes of action of the rho locus are duscussed with reference to this and pre-existing data on the rho phenotype.