A series of 5-[(arylcarbonyl)amino]- and 5-(arylcarbamoyl)pyridine-2-carboxylic acids has been prepared and tested for activity as inhibitors of the enzyme prolyl 4-hydroxylase (EC 1.14.11.2). All the analogues prepared were inhibitors of the enzyme in vitro, the best compounds being equipotent with the known inhibitor pyridine-2,5-dicarboxylic acid (9). Like 9 these amidic analogues were not active in a cultured embryonic chick tendon cell model, considered to be a predictor of in vivo activity. The activity of the amides is not consistent with the model described for the mode of action of 9 with the enzyme and aspects of this are discussed.