Using the techniques of bioassay the presence, production, and importance of prostaglandins (PG) in human bladder muscle were investigated in vitro. Low basal tissue levels of PG were shown. E series PG were released by isolated bladder muscle strips in organ bath experiments. The spontaneous activity and PG release of these strips could be markedly reduced by the addition of indomethacin. Incubation of bladder muscle with the PG precursor, arachidonic acid, resulted in the synthesis of substantial quantities of PG.