Mature female Sprague-Dawley (SD) and Long-Evans (LE) rats were ovariectomized (OVX), fitted with indwelling atrial catheters and given a single sc injection of either 25 or 100 micrograms polyestradiol phosphate (PEP); seven days later blood samples were withdrawn at two hour intervals from 1100 to 2100 hours to detect the presence of an afternoon surge of prolactin (PRL). Other groups of OVX rats of both strains also treated with PEP and catheterized as above were sampled before and at 2, 5, 10 and 30 min after iv administration of 1 microgram synthetic thyrotropin releasing hormone (TRH). Pituitary (AP) and uterine weights were determined following sacrifice one day after TRH treatment. Separate groups of OVX rats of both strains treated with 100 micrograms PEP were decapitated 7 days later and each AP was removed and homogenized. The AP homogenates and plasma samples were assayed for PRL by radioimmunoassay. Rats of both strains had afternoon PRL surges and in both strains the magnitude and/or duration of the surges were enhanced by the higher dose of PEP. However, within each PEP dose LE rats released significantly more PRL during the surge than did SD rats. Rats of both strains also released PRL in response to TRH and this response was enhanced in both strains by the higher of the two doses of PEP. However, there were no differences between the strains at 25 micrograms PEP and at 100 micrograms PEP SD rats released significantly more PRL to TRH than did LE rats. Pituitary weight and PRL concentration were not different between the strains at either dose of PEP but LE rats had significantly heavier uteri at both doses of PEP compared to SD rats. These data not only show that strain differences exist in estrogen-induced or mediated PRL release in the rat but also indicate that the differences are not uniform. This latter observation suggests that the estrogen-induced mechanisms examined in this study are for the most part independent of each other.