This study was designed to investigate the effect of anterolateral hypothalamic deafferentiation (ALHD) and medial preoptic area (MPOA) lesions on plasma LH levels in the long term ovariectomized rat. The deafferentations were carried out with a Halasz-Pupp knife (radius of 1.5 mm and height of 2.0 mm) and the MPOA lesions with a platinum electrode. Sham treated and an intact group served as controls. Blood samples were obtained from the jugular vein under light ether anesthesia before and at 1, 2, 4 and 6 weeks after brain surgery. After the sixth week sample all rats were treated with 50 micrograms of estradiol benzoate (EB) and two days later blood samples were collected during the morning and afternoon. Hypothalamic deafferentation resulted in a more significant (p less than 0.01) drop in plasma LH levels in one half of the group (ALHD-1) than in the other half (p less than 0.05) (ALHD-2) when compared to the controls. Treatment of the controls with EB resulted in a significant (p less than 0.01) depression of LH levels in the morning and an LH surge during the afternoon. EB also resulted in a suppression (p less than 0.01) of LH levels during the morning in all of the ALDH rats; however, only the ALHD-1 group had an LH surge during the afternoon following EB. Plasma LH levels in the ALHD-2 remained suppressed during the afternoon after EB treatment. Lesions in the MPOA had no effect on plasma LH levels at 1 to 6 weeks when compared to controls. Treatment of the MPOA lesion group with EB resulted in a significant (p less than 0.01) drop in plasma LH levels during the morning as well as the afternoon. These data suggest that the fibers that are critical for the control of tonic and phasic LH secretion enter the medial basal hypothalamus laterally and that the deafferentations carried out here were selective in interrupting fibers involved with tonic LH secretion in some rats and those involved with the phasic secretion in others. These data also suggest that the MOPA components involved with tonic LH secretion are separate from those controlling phasic LH secretion.