This study examined the effects of prostaglandin-F2alpha (PGF2alpha), GnRH, and their interactions on steroidogenesis in human granulosa-luteal cells (GLCs). Human GLCs collected from in vitro fertilization patients were cultured for one or eight days, followed by a 24-h treatment period, after which media were collected and radioimmunoassayed for progesterone and estradiol. In the first experiment, GLCs were treated with vehicle, PGF2alpha (10(-9) M), GnRH (10(-6) M), or PGF2alpha plus GnRH, with or without hCG (1 IU/ml). Neither PGF2alpha nor GnRH alone had a significant effect; however, the combination of PGF2alpha plus GnRH significantly stimulated steroidogenesis. Similarly, co-application enhanced the luteolytic effects of PGF2alpha. In a second experiment, PGF2alpha and GnRH concentration-response curves were crossed into a matrix of 49 separate treatments. Responses were plotted in three-dimensions and as two-dimensional "slices" that were analyzed statistically. In the presence of high concentrations of GnRH (10(-6) M), PGF2alpha stimulated progesterone production in a biphasic manner, as middle concentrations significantly stimulated (10(-9) M) whereas low and high concentrations did not. In the presence of middle concentrations of PGF2alpha (10(-9) M), GnRH significantly stimulated progesterone production in a linear concentration-dependent manner. Similar complexities were seen with respect to estradiol response. Thus, in the human GLC, GnRH potentiates the luteolytic effects of PGF2alpha, while it acts as a permissive factor for the luteotropic effects. Furthermore, we have revealed the complex interaction of these hormones using a three-dimensional experimental design.