Rats subjected to porto-caval shunt present, even after a rather long period since the operation (5 months), a moderate atrophy of the testicles. This phenomenon occurs because of a reduced gonadotropic activity of the pituitary gland, consequent upon a more or less increased inhibition of the hypothalamic GTH-RF production. The abnormal depression of the hypothalamic activity is due to a scarce metabolism of the sexual hormones following the operation.