The steroidogenic responsiveness of human thecal tissue to different doses of LH was investigated in vitro in relation to the health of the follicle and to the responsiveness of stromal tissue. The results show that small incremental increases in LH, over a low range of concentrations (1 to 10 ng/ml), markedly increased the thecal output of androstenedione from healthy and/or atretic follicles. Theca from healthy follicles were also stimulated to increase their output of progesterone and estradiol in response to small increases in LH whereas theca from atretic follicles produced more variable amounts of progesterone and were unable to generate estradiol. In contrast, relatively high concentrations of LH (50 ng/ml) reduced the total steroid output from the theca of both healthy and atretic follicles while 'switching on' a low level of steroidogenesis in stromal tissue. These data suggest that the steroidogenic response of thecal tissue is related to the mass of tissue (i.e., the size of the follicle), the health of the follicle and the amount of LH to which it is exposed.