The effects of pentobarbital (PBTL) on pulsatile LH release in ovariectomized rats were studied and characterized. Unanesthetized rats were first bled continuously (50 μl whole blood every 5–6 min) through indwelling right atrial cannulae for 1½ h prior to PBTL injection (35 mg/kg). Animals were then bled for various periods of time during PBTL-induced anesthesia, as well as for 1½–2 h after recovery of the ½ righting reflex. Whole blood was analyzed for LH by radioimmunoassay. PBTL administration initially inhibited pulsatile LH release for 1¼–1½ h, after which episodic release resumed. For the remaining duration of anesthesia (3–3½ h) episodic LH release was characterized by decreased mean blood LH levels, a decreased rate and magnitude of increase in blood LH levels for individual episodes, and a lengthened periodicity between pulses. However, though reduced, episodic LH release remained stable despite the continued influence of anesthesia. In the 1½–2 h following recovery of the ½ righting reflex, only the periodicity of LH release returned to a pre-anesthesia interval. Mean blood LH levels and the rate at which blood LH levels changed during individual pulses remained reduced, and a slight fall occurred in the extent of the increase in blood LH levels during single LH episodes. The influence of PBTL on those brain regions responsible for controlling pulsatile LH release is therefore of longer duration than the effects of this anesthetic on central nervous system (CNS) areas, which when affected, results in loss of the ½ righting reflex.