In the present chapter the behavioral and neuroendocrine alterations accompanying pregnancy and lactation will be discussed. It will be shown that many are dependent on the innate level of emotionality of the rats. In late pregnancy the level of anxiety, as measured on the elevated plus-maze is increased in rats with both high and low level of innate anxiety-related behavior, whereas lactating rats display less anxiety in such tests and higher degrees of aggressive behavior in tests for agonistic behavior. There is a dramatic reduction in the responsiveness of the hypothalamo-pituitary-adrenal (HPA) axis to various physical or emotional stimuli in both pregnant and lactating rats. This appears to be due to changes throughout the HPA axis. Oxytocin has been implicated in the control of the axis at this time, but the inhibitory action of central oxytocin on ACTH or corticosterone secretion seen in virgin female rats is not evident during pregnancy and lactation. However, central oxytocin is involved in the regulation of emotionality at this time. In addition to its anxiolytic effect, prolactin, acting at brain prolactin receptors, seems to exert an inhibitory effect on HPA axis responsiveness. At the time of parturition, the HPA axis is not stimulated by parturition-related stimuli and is under strong inhibition by endogenous opioids as revealed by the application of the opioid receptor antagonist naloxone.