We examined several of the possible reasons for the observed effects of high bromide levels in the organism of lactating rat dams on the development of the sucklings. Analysis of mother's milk showed an easy permeation of bromide ions, the amount of which in the samples of rat milk being dependent on the bromide concentration in the drinking water taken by the dams. With 5 g bromide per liter, about 54 % of chloride ions were replaced by bromide, the sum of the molar concentrations of both halogens being unchanged.