Effects of a high bromide intake in lactating rats on the performance of the dams and on the prosperity of their young were studied. In the dams, two marked consequences undoubtedly caused by high bromide intake were observed: stagnation in the extent of diet and water consumption in the course of the lactation period, and a conspicuous drop in the production rate of mother's milk. A very high intake of bromide in the mothers in the course of the nursing period (about 220 mg Br-/d per dam) also caused a marked decrease in the body weight increments in their suckling young. Only about one-half of these young survived and their general condition was very poor. It is suggested that one of the possible reasons for the observed marked decrease in the production of mother's milk in dams with high bromide intake could be a decreased stimulation of the mammary glands as a consequence of reduced consumption of mother's milk by the suckling. Bromide ions ingested by the dams easily moved into the rat milk. Via mother's milk, bromide was transferred in a large extent to the suckling. The amount of bromide in mother's milk depended on the bromide concentration in the drinking water taken by the dams. With the addition of 5 g bromide per liter (providing the mean daily bromide dose of 220 mg), bromide ions replaced about 54% of the chloride in the milk. A rise in the concentration of both halogens caused also an increase in the concentration of sodium in mother's milk. The exact mechanism(s) of bromide interference with postnatal developmental processes in the young remain(s) unclear.