This paper reviews our growing understanding of the external environmental and the internal neuroregulatory mechanisms for the infant rat's vocal response to abrupt isolation from nest, littermates and mother. Interactions of the rat pup with specific components of its animate and inanimate environment have been identified and found to exert a tonic regulatory action on the neural systems mediating ultrasonic vocalization (USV). In isolation, withdrawal of these regulators produces an acute outburst of calling. Changes in rate of ultrasonic calling in response to administration of receptor agonist and antagonist agents generally correspond to the effects of those agents on measures of anxiety in adult animals and in human clinical populations, suggesting a conservation of neural substrates for anxiety in evolution and in development. The inhibition of isolation calling in response to predator cues and its potentiation by brief exposure to maternal cues, represent novel forms of regulation with implications for neuroendocrine development and for the neurobiology of early cognitive-emotional processes.