Most mammalian infants cry when distressed by separation from the mother. In rat pups, the rate of ultrasonic vocalization is considered to be an index of the degree of discomfort. The two and a half week period during which rat pups produce ultrasounds in response to social isolation encompasses the postnatal period of rapid growth and development of many neural systems including the opioids. While other neurotransmitter systems are also involved, morphine is known to quiet and naltrexone exacerbate isolation distress in 10-day-old rat pups. In the proposed series of experiments we seek to determine the neurobiological substrates that mediate this reaction and to assess the relative impact of mu, delta and kappa opiate receptors in the modulation of isolation distress. Pups of three ages will be tested alone in a novel environment. The rates of ultrasonic vocalizations, activity and various other behaviors will be monitored after administration of specific opiate receptor agonists, to ascertain the ontogeny of opioid involvement in the expression and suppression of isolation distress. When the pup has either dam or littermate as a companion, there is a dramatic decrease in the rate of vocalization. This companion effect is naltrexone reversible. In a second series of experiments, we shall determine which of the antagonists, specific to opiate receptor types, is effective in blocking the comfort effect at the three test ages. MacLean (1985) characterizes the separation call as being perhaps the earliest and most basic mammalian vocalization and Bowlby (1969, 1973) speculates that separation anxiety is not learned by a child but is an evolved protest mechanism, released by the mother's absence. Vocalization summons the mother and elicits retrieval. The ultrasonic cry of the isolated pup, therefore, may provide a model for the intense anxiety common to mammalian young when faced with the sudden loss of familiar social figures. If we can understand the neurochemical substrates of this "normal" manifestation of early anxiety, it may provide us with clues to the etiology of childhood separation disorders in humans.