Transforming growth factor alpha (TGF alpha) is implicated as a paracrine growth factor in the regulation of human granulosa cell function. To investigate this further, we have examined the actions of TGF alpha on the basal and follicle-stimulating hormone (FSH)-stimulated aromatase activity of human granulosa cells to determine how this growth factor influences oestrogen biosynthesis in the follicle. Granulosa cells from women having in-vitro fertilization during untreated or gonadotrophin-stimulated cycles were cultured for 1-6 days in the presence or absence of FSH or TGF alpha at a range of doses. Aromatase activity, expressed as oestradiol production, was determined after culture during a 3 h test period. After 2 days, TGF alpha (1-300 ng/ml) decreased basal and FSH-stimulated aromatase activity in a dose-dependent manner (ED50 = 3 ng/ml). In contrast, after 4 days, TGF alpha enhanced both basal and FSH-stimulated aromatase activity. Repeated experiments revealed a consistent pattern of inhibition on day 2, which was more marked in the presence of FSH (reduction by 30.6 +/- 9.1%, mean +/- SEM; n = 14; P < 0.01), and stimulation on day 4 in both the absence (increased by 61.4 +/- 20.6%, mean +/- SEM; n = 6; P < 0.05) and presence of FSH (increased by 36.0 +/- 15.2%, mean +/- SEM; n = 8; P < 0.05). The results provide further evidence that TGF alpha is a paracrine factor in the control of oestrogen biosynthesis, but the actions can be either inhibitory or stimulatory depending on the duration of exposure.