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1. A study was made of the effect of an exteroceptive stimulus, produced by a team of lactating rats and of litters while suckling, on the pituitary melanocyte-stimulating hormone (MSH) activity of a group of lactating rats.

2. After being isolated for 9 hr from their litter, one group of lactating rats was killed after being allowed to nurse for 30 min, another group was subjected to the exteroceptive stimulus for 15 min and then killed. Control rats were killed after 9 hr of isolation from the litter without being subjected to either suckling or the exteroceptive stimulus.

3. The group of rats which had suckled their young showed a pituitary MSH activity equal to 35 ± 2·29%, and the group exposed to the exteroceptive stimulus an activity equal to 59·5 ± 3·16% of that in the control animals.

4. When deaf rats were used, only the suckled mother showed a decrease of the pituitary MSH activity which was similar to that obtained with the normal suckled mother, indicating that the exteroceptive stimulus depended on hearing for its effect.

5. A light dose of sodium pentobarbitone was sufficient to block the effect of suckling and the auditory stimuli on the pituitary MSH activity.

6. The role of the central nervous system in this phenomenon and the possibility that MSH is implicated in the process of lactation are discussed.