The influence of follicular maturation on steroidogenesis and steroid metabolism by isolated Japanese quail granulosa and theca cells was examined. When stimulated with LH, granulosa cells of the largest follicle (F1) responded with a sixfold increase over unstimulated progesterone levels, whereas progesterone production in cells of F3 less than doubled even when maximally stimulated. Forskolin stimulated progesterone synthesis in both F1 and F3 granulosa cells, but its effect was less pronounced than that of LH. Furthermore, F1 cells metabolized 25-hydroxycholesterol to a greater extent than did F3 cells. There was no appreciable metabolism of [3H]progesterone by granulosa cells. Theca cells from the smaller follicles (F3-F5) responded to LH stimulation with greater estrogen and androstenedione production than theca cells from F1. [3H]Progesterone was metabolized mainly to androstenedione in theca cells. Thus, the overall pattern of in vitro steroidogenesis in quail granulosa cells is similar to that described for the chicken and turkey even though the quantitative differences in the steroidogenic capacity between developing and mature follicles are more striking in the quail. Furthermore, although the LH-stimulated androstenedione and estrogen production appears similar in developing quail and chicken theca cells, the profile of [3H]progesterone metabolism is different in quail theca cells from that found previously in chicken theca cells.