The mean plasma total tryptophan concentration of 13 long-standing rheumatoid arthritis patients was found to be lower than that of seven nonrheumatoid control subjects, but the plasma nicotinic acid concentration was unchanged. In the rheumatoid patients the urinary excretion of the tryptophan metabolites, kynurenine, xanthurenic acid and 3-hydroxyanthranilic acid, was increased several fold, but the excretion of N-methylnicotinamide was normal. These findings are discussed in relationship to the dietary intakes of tryptophan, nicotinic acid and pyridoxine, the effect of antirheumatoid drugs on plasma tryptophan and liver tryptophan pyrrolase, and the requirement of rheumatoid patients for pyridoxine.