In vitro metabolism of pregnenolone (P 5 ) as well as production of 17b-estradiol (E 2 ) were studied in uteri of untreated and luteinizing hormone (LH)-treated mice that had been ovariectomized (OVX) at late-diestrus stage. In the uteri of untreated mice, [ 3 H]pregnenolone was shown to be metabolized to D 5-components such as 17a-hydroxypregnenolone (17a-P 5 ) and dehydroepiandrosterone (DHEA), whereas LH treatment resulted in significant increases in the formation of progesterone (P 4 ), 17a-hydroxyprogesterone (17a-P 4 ), androstenedione (AD) and testosterone (T). This was assessed by thinlayer chromatography (TLC) and high-performance liquid chromatography (HPLC). The content and release of P 4 was shown to be stimulated by LH. Trilostane, an inhibitor of 3b-hydroxysteroid dehydrogenase (3b-HSD), inhibited LH-induced P 4 synthesis and its release in a dose-dependent manner. A considerable increase in [ -3-oxosteroids. In vitro effects of LH on the synthesis and metabolism of P 4 , as well as on the stimulation of aromatase activity, were more pronounced in the uterine tissue of LH-primed OVX mice. Thus the results of the present study indicate that, under specific conditions, the uterus of the mouse behaves like steroidogenic tissue. Its prompt response to LH reveals the probable physiological relevance of the existence of LH receptors of high binding affinity in the uterine tissue of the mouse, as reported earlier.