The synthesis of the pure diastereoisomer of 16-methoxy-16-methyl-PGF2 alpha, -PGE2, and -PGE1 is described. The absolute configuration of C-16 was established by chemical methods, while the absolute C-15configurations of the diastereoisomers were assigned tentatively on the basis of their chromatographic behavior and NMR spectra. The synthetic prostaglandin analogues were evaluated for antisecretory