1.Adult specimens of Ascaris lumbricoides of similar weights were incubated under nitrogen for 24 hours in a synthetic medium with 1 mM 2,2′-dipyridyl.2.Under these conditions, the viability of the parasites was not affected as evidenced by the amount of ATP in the whole sample and the mobility after mechanical stimulus.3.Incorporation of [14C]proline into non-collagenous proteins in the body wall and cuticle was reproducibly higher in 2,2′-dipyridyl-treated specimens than in untreated worms. Synthesis of collagenous hydroxyproline was inhibited in the cuticle and, to a greater extent, in the muscle layer.4.After transferring the specimens into a fresh medium enriched with 0·1 mM ferrous ions and incubated under 70% oxygen, the muscle collagen remained underhydroxylated. The synthesis of hydroxyproline, however, was almost completely normalized in the cuticle collagen.5.We interpret the data as further evidence of the existence of at least two different enzymes hydroxylating collagenous proline, one located in the subcuticle and the other in the muscle layer of Ascaris lumbricoides.