The administration of small amounts of mercuric compounds to pregnant rats, injected with 75Se in the form of selenite, markedly increased their retention of 75Se and simultaneously decreased the 75Se content in their foetuses. The 75Se concentration in the blood and liver of foetuses from these rats was much lower than in the blood and liver of foetuses from control mothers injected with [75Se]selenite only (or receiving zinc chloride instead ofmercuric chloride). Retention of 75Se in the maternal organs and the passage of 75Se into the foetuses was partially dependent on the dose of mercury. In the doses used in these experiments, administration of mercuric salts did not result in foetal death nor in any significant effect on the weight of the offspring. In lactating rats, the administration ofmercuric salts had a similar effect on the passage of 75Se into the milk. This resulted in a much smaller 75Se content in the newborn and suckling young of mothers exposed to small amounts of mercuric compounds during pregnancy or lactation than in control animals.