Immature and adult female rats were bilaterally implanted in the medial preoptic area (MPOA) or hypothalamic ventromedial-arcuate region (VMAR) with the dopamine (DA) antagonist, pimozide, or the DA agonist, apomor-phine, and the sensitivity to the LH-inhibiting effect of a subcutaneous injection of estradiol benzoate (EB), the onset of puberty and ovarian cyclicity were investigated. Diminution of the inhibitory effect of EB on LH secretion was recorded in ovariectomized immature and adult females implanted in the MPOA with pimozide. This response was not obtained in rats implanted in the VMAR. In contrast, medial preoptic, but not intrahypothalamic, implantation of agar pellets containing apomorphine resulted in enhanced sensitivity to estrogen both prepubertally and during the ovarian cycle. The sensitizing effect of apomorphine was completely prevented in prepubertal rats by pretreatment with EB for 6 days. Bilateral implantation of pimozide-agar pellets in the MPOA of 28-day-old intact females induced significant advancement of the onset of puberty, whereas sexual maturation was not affected by daily subcutaneous injections of the DA antagonist from day 28 till the day of vaginal opening. In adult 4-day-cyclic rats fitted with guide cannulae, the forthcoming ovulation was delayed for about 7 days as compared to the controls implanted with agar if apomorphine was placed in the MPOA from the morning of metestrus to the morning of diestrus. Similar implants located in the VMAR were ineffective in this regard. The results suggest that: (1) low DA activity in the MPOA reduces and high activity enhances the sensitivity of the negative estrogen feedback in immature and adult female rats; (2) the medial preoptic DA activity seems to be involved in the control of female puberty and of the ovarian cycle, and (3) a relationship may exist between the estrogen-induced prepubertal and preovulatory desensitization of the negative estrogen feedback and the DA neurotransmission in the MPOA.