Potassium thiocyanate, given to the drink of pregnant mothers provoked a body weight decrease of 14 day-old mice (-18%), without perturbations of cerebrum and cerebellum weights. The reduction of pups' body weight could be explained by a decrease of plasmatic thyroid hormone levels: FT3 and FT4, (-54; -57%) in youngs and (-66; -49%) in their mothers. A defect of thyroid hormone levels, observed in young mice, provoked a decrease of cerebrum and cerebellum protein concentrations (-15 and -13%) respectively. Modifications of biochemical parameters confirmed the histological aspects of hypothyroid mice cerebellum with purkinje cells incompletely differentiated. When thiocyanate ion was removed from mothers' drink, we have noted in pups a partial return of some biochemical parameters (iodine thyroid contents, protein concentrations of cerebrum and cerebellum) and of cerebellum histological aspects. While thyroxinemia (FT4) and plasmatic FT3 levels were completely restored and even exceeded control values, which were necessary for central nervous system maturity of young.