Injection of interleukin-1 (IL-1) into the third cerebral ventricle (3V) of conscious rats increases plasma prolactin (PRL) concentration. Nitric oxide (NO) is involved in control of corticotropin-releasing factor (CRF) and luteinizing hormone-releasing hormone (LHRH) release. Consequently, we evaluated its role in the PRL-releasing action of IL-1. In the present experiment, NG-mono-methyl-L-arginine (NMMA) (1 mg in 5µl of 0.9% NaCl (saline)], an inhibitor of NO synthase, or 5µl of saline was microinjected into the 3V of conscious, castrate male rats and blood samples were removed from jugular catheters just prior to and at 10-min intervals after injection. A second injection of NMMA or saline was given 60 min after the first. Five minutes after the injection of NMMA or saline, IL-1Α(0.6 pmol in 2µl saline), or an equal volume of saline, was injected into the 3V. Plasma PRL concentrations were increased within 10-20 min after injection of IL-1Αand a second pulse of PRL usually occurred at 60-70 min following its injection. The maximal increase in plasma PRL from the initial value in the animals injected with IL-1Αwas highly significant, whereas there was no significant increase in the animals injected with NMMA plus IL-1Αor in the animals injected with saline or NMMA. The area under the plasma PRL curve was significantly elevated in the animals injected with IL-1Αabove that of rats injected; with NMMA plus IL-1Αduring the first hour after injection. NMMA transiently slowed the decrease in plasma PRL which occurred after intraventricular injection of saline, but otherwise had no effect. We conclude that IL-la releases PRL by activation of NOergic neurons in the hypothalamus which stimulate the release of PRL-releasing factors or inhibit the release of PRL-inhibiting factors resulting in the stimulation of release of PRL from the lactotrophs. It is unlikely that this effect occurs via a pituitary action of IL-1Α.