Salmonella-associated arthritis of rats was used as an experimental model for in vivo evaluation of prostaglandins in chronic inflammation. Since the arthritic lesions were localized mainly in the hind paws, femorla vein plasma measurements of prostaglandins were made to estimate the amounts of PGE1, PGE2 alpha, 6-keto-PGF1 alpha and TXB2 produced during the progression of the arthritic disease. PGE1 production was significantly increased in the early phase of infection but these changes were inversely related to the joint swelling in the later stages of the disease. Changes in products of arachidonic acid (20:4 omega 6) were opposite to those observed in PGE1, a product of dihomo-gamma-linolenic acid, (20:3 omega 6), in that measurable levels of these chemicals were lower in the early phase of infection but there was a progressive increase in each compound during the chronic phase of inflammation. Concentrations of PGE2, PGF2 alpha, 6-keto-PGF1 alpha and TXB2 increased proportionately to the severity of the arthritic lesion.