Compounds of formula (I): ##STR1## (wherein n is 0 or 1, R.sup.1 and R.sup.2 are hydrogen or alkyl, one of A.sup.1 and A.sup.2 is --Z--Y and the other is --W--COOH, where W and Z are alkenylene or alkylene and Y is imidazolyl or pyridyl, and the broken lines represent two single bonds or one single bond and one double bond) and their salts, esters and amides have the ability to inhibit the synthesis of thromboxane A.sub.2 and hence are useful in the treatment or prophylaxis of thrombotic conditions. They may be prepared by introducing an imidazolyl or pyridyl group into the corresponding compound in which Y is replaced by an active group or atom.