The follicular fluid is an important milieu for the growing and maturing oocyte and granulosa cells. In this study we investigated: (1) the properties of gonadotrophin-binding sites in the supernatant fraction of porcine follicular fluid (pFF) and compared them with those of membrane-bound receptors, and (2) the relative changes that occur in pFF and granulosa cell receptor-binding activity following hormone priming of gilts. 125I-Labelled human chorionic gonadotrophin (hCG) and 125I-labelled ovine FSH (oFSH) binding to particulate and supernatant fractions of pFF were hormone-specific and saturable. The concentration of 125I-labelled hCG-binding sites was roughly 50-fold higher in particulate than in supernatant fractions of pFF. However, 30-40% of the total 125I-labelled hCG-binding activity in pFF was present in the supernatant fraction of commercial batches of pFF. 125I-Labelled oFSH binding to pFF membranes was markedly higher than to supernatant fractions. Binding of 125I-labelled hCG and 125I-labelled oFSH to granulosa cells and supernatants of pFF showed a time-dependent variation in response to hormone priming. The results suggest that gonadotrophin-binding sites in the supernatant fraction of pFF have properties similar to those of their membrane-bound counterparts. 125I-Labelled hCG-binding activity in the supernatant fraction of pFF was shown to be more stable than detergent-solubilized LH/hCG receptors, even in glycerol-preserved preparations. Based on a number of criteria, we have speculated that pFF may have components which may be similar in structure to the extracellular domain of the LH/hCG receptor.