Contractions of isolated iris sphincter muscles were measured in response to several free fatty acids, hydroperoxy and hydroxy derivatives of 20:3(n-3), 20:3(n-6) and 20:4, PGH2, and the epoxymethano methano analogs of PGH2. The free acids of prostaglandin precursors elicited comparatively strong contractions, hydroperoxy and hydroxy acids gave intermediate and nonspecific response whereas nonprostaglandin precursor acids elicited little response. PGH2 was 100 to 1000 times more effective than arachidonic acid or the epoxymethano analogs. The latter compounds inhibited the production of contractions by PGH2. These results allow an interpretation that the iris sphincter muscle contains an active thromboxane synthase and receptors for endoperoxide and thromboxane that initiate contraction.