Pulsatile luteinizing-hormone releasing hormone (LHRH) secretion was measured from the mediobasal-suprachiasmatic-preoptic (MBH-SCN-POA) region of the hypothalamus from fetal baboons (Papio anubis) at midgestation (day 100; term = day 184). The entire MBH-SCN-POA (48 ± 5 mg) was obtained between 1100 and 1200 hours and was immediately placed in icecold phosphate buffer (pH 7.4). The MBH-SCN-POA units were halved at the midline and superfused in parallel at 37.5°C for 5 hours. Then 500 μl of superfusate was collected at 10-minute intervals, and LHRH concentration was measured by radioimmunoassay using the Chen-Ramirez antibody. In fetuses of untreated baboons (N = 3), LHRH pulse amplitude (mean ± SE) was 16.0 ± 4.2 pg, with a period of 30 ± 1 minute; the average 10-minute output of LHRH was 9.4 ± 2.0 pg. In fetal baboons in which the hormonal milieu in the mother was modulated by androstenedione treatment of midpregnancy (N = 3), average LHRH pulse amplitude was 1.7 ± 0.3 pg, with a period of 33.5 ± 4.9 minutes; the average 10-minute output of LHRH was 1.2 ± 0.2 pg. Collectively, these data suggest that as early as midgestation, fetal baboons secrete LHRH in vitro in a pulsatile fashion and with a periodicity of 30–35 minutes. In addition, the decrease in LHRH pulse amplitude and the average 10-minute LHRH output (P < .01, P < .05) in tissue from fetal baboons of mothers in which the normal pattern of steroidogenesis is altered suggest that the output of LHRH systems in the fetus is sensitive to changes in the maternal hormonal milieu.