Extracellular unitary records were obtained from neurons in the ventromedial hypothalamic nucleus (VMH) of very young (1-25 days of postnatal age) and adult rats. Spontaneous unitary activity and evoked responses to both external (somatic, gustatory, and olfactory) and internal sensory (systemic administration of hypertonic saline and glucose solutions) stimulation were determined in order to assess the functional development of VMH neurons and their afferents. The basic electrophysiological characteristics of VMH neurons were established prenatally. From the date of birth, many VMH neurons had: spontaneous action potential generation; evoked responses to external or internal sensory stimulation; and convergent sensory inputs. In contrast, the major developmental change in the neurophysiological properties of VMH neurons was the diminution with increasing age of the convergence of external and internal sensory influences. This developmental 'fine-tuning' of a complex functional feature of VMH neurons is important because the maturation of convergence coincides with a 'critical period' of VMH ontogenesis demonstrated in behavioral and experimental brain damage reports.