In order to study the fate of human follicle-stimulating hormone (FSH) when hormone binds to its receptor, a quick biochemical method that can differentiate between the surface-bound and internalized hormone was used to determine the internalization induced by FSH in cultured both porcine granulosa cells and Chinese hamster ovary (CHO) cells expressing recombinant porcine FSH receptor. The results showed that FSH was slowly internalized, and the internalized radioactivity (acid resistant) reached a peak 10-12 h after addition of 125 I-hFSH. It was suggested that FSHR do not get internalized rapidly under physiological circumstances precisely because the appropriate sequences are absent. In order to study the fate of human follicle-stimulating hormone (FSH) when hormone binds to its receptor, a quick biochemical method that can differentiate between the surface-bound and internalized hormone was used to determine the internalization induced by FSH in cultured both porcine The results showed that FSH was slowly internalized, and the internalized radioactivity (acid resistant) reached a peak after 10-12 h after addition of 125 I-hFSH. It was suggested that FSHR do not get internalized rapidly under physiological cases precisely because the appropriate sequences are absent.