The immune expulsion of gastrointestinal nematodes is impaired during lactation by the inhibition of the lymphocyte mediated component. As a result the population of worms may increase. At the same time, the fecundity of female worms in previously immune hosts may increase to approach that of worms in fully susceptible animals. There is good evidence that these events are primarily endocrinal in origin but although prolactin is probably involved, certain anomalies exist. These may reflect differences in the secretion pattern of the hormone in different phases of reproduction or they may indicate an interrelationship with a second hormone, for instance, adrenal glucocorticoids.