The prostaglandins, PGE1 and PGE2, effectively inhibit the IgE- and IgGa-mediated release of SRS-A in the rat in a dose-response fashion. Structure-function studies suggest that the carbonyl group in the 9 position, the position of a double bond in the cyclopentane ring, and either the degree of saturation of the cyclopentane ring or the presence of the hydroxl group in the 11 position, or both, are required for optimal inhibitory activity. Indirect evidence did not establish that prostaglandin inhibition of SRS-A release is mediated through an increase in tissue cyclic AMP levels and no synergism between PGE1 and diethylcarbamazine or diso-dium cromoglycate was demonstrable. © 1971, SAGE Publications. All rights reserved.