Gonadotrophin releasing hormone (GnRH) exhibits both stimulatory and inhibitory effects on the ovarian tissue in the rat. For the action of GnRH on the pituitary, the mode of administration is of utmost importance. The binding characteristics and regulation of the GnRH receptors found in the ovary are very similar to those of the pituitary. The aim of the present study was to compare the steroidogenic response of superfused, preovulatory rat follicles to continuous and pulsatile administration of a GnRH analogue (GnRHa). The results obtained with a single pulse of luteinizing hormone (LH) served as a control. Repeated pulses (1 pulse X h-1) or continuous administration of GnRHa was found to be more effective than a single pulse of GnRHa. The effect of GnRHA, irrespective of the mode of administration, was most pronounced on the secretion of 20 alpha-OH-progesterone (20 alpha-OHP). Also the release of testosterone (T) and oestradiol (E2) was stimulated but to a lesser extent. In comparison to LH, the GnRHa response was delayed and without detectable release of cyclic AMP (cAMP). These results illustrate that pulses of GnRH stimulate the steroidogenesis in preovulatory follicles. In contrast to the effects of GnRHa on the pituitary, the modes of administration seem to be of less importance for the stimulation of the rat ovary.