Immunoreactive prolactin (ir-PRL) has been identified in the cerebrospinal fluid (CSF) and brain of the male and female rat. In this study we determined the concentration of ir-PRL in the CSF and hypothalamus under conditions known to increase or decrease serum PRL. Hypophysectomy (60 days) significantly decreased the concentration of ir-PRL in the CSF of male (4.9 +/- 0.7 vs. 3.0 +/- 0.3 ng/ml) and female (5.8 +/- 0.9 vs. 3.1 +/- 0.5 ng/ml) rats. However, the effect of long-term hypophysectomy on hypothalamic ir-PRL was gender-dependent. That is, in the male rat hypophysectomy did not affect the content of ir-PRL in the female rat, long-term hypophysectomy decreased the content of ir-PRL in the median eminence, ventral hypothalamus, and dorsal hypothalamus 37, 40, and 47%, respectively. Estradiol replacement to the hypophysectomized female rat normalized the content of ir-PRL in the median eminence (96 +/- 5.8 to 131 +/- 9.6 ng/mg protein), ventral hypothalamus (11 +/- 0.6 to 16.0 +/- 1.1 ng/mg protein), dorsal hypothalamus (4.7 +/- 0.4 to 8.6 +/- 0.4 ng/mg protein), and the concentration ir-PRL in the CSF (2.5 +/- 0.3 to 4.6 +/- 0.4 ng/ml). In intact female rats, administration of haloperidol induced a marked hyperprolactinemia, and significantly increased CSF ir-PRL (5.1 +/- 1.5 vs. 18.0 +/- 3.8 ng/ml). However, in the same rats, the content of ir-PRL in the median eminence was significantly decreased while the ir-PRL content in the ventral hypothalamus and dorsal hypothalamus was unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)