Experiments were conducted in vivo to investigate further if the control of the pulsatile plasma LH phenomenon in ovariectomized (OVX) rats is located in the brain or in the adenohypophysis. Luteinizing hormone releasing hormone (LHRH) was infused at a constant rate (2–100 ng/h) through an indwelling venous cannula in unanesthetized, unrestrained OVX rats. Blood samples were collected at 5-min intervals through a second venous cannula prior to and during LHRH infusion for subsequent radioimmunoassay of plasma LH. LHRH infusion at 12.5, 50 and 100 ng/h did not interfere with the magnitude or the periodicity of LH pulses in plasma but the range within which plasma LH fluctuated was elevated. Phenobarbital (75 mg/kg BW; i.p.) blocked the pulsatile plasma LH and maintained the plasma LH nearly constant at reduced levels. Pulse i.v. injections of LHRH but not constant rate i.v. infusions restored pulsatile LH patterns in phenobarbital-treated OVX rats. The results are consistent with the view that pulsatile LHRH release is responsible for the pulsatile nature of plasma LH in OVX rats. The results do not support the concepts of a short-loop feedback of LH or an ultra-short-loop feedback of LHRH on LH secretion at least on an acute basi