The multiple unit activity of the rat neural lobe was investigated to determine how accurately it represents the neural events accompanying the actual release of hormones by the neurohypophysial system. Microelectrode recordings were done in normal and midbrain-transected rats to identify the unit activity belonging to the neural lobe and to characterize its activity. Following the intracarotid bolus injection of hypertonic saline, the neural lobe responded with several complex patterns. Each one of them was made up of one or more of five different possible events: initial burst, short inhibition, delayed bursting activity, prolonged increase and prolonged inhibition. The neural lobe response usually lasted long after the brief osmotic stimulation was over. Some of the patterns of unit activity found in the neural lobe reflected the reported pulsatile patterns of hormone release following hypertonic stimulation revealed by sequential measurement of plasma vasopressin levels. The rats with midbrain transection did not show the short inhibition event, probably as a result of the removal of information impinging on the system from peripheral sources coming from or through the mesencephalon.