1 Nine patients with rheumatoid arthritis or non-inflammatory backache were given soluble aspirin (65 mg/kg body weight) daily. There was no significant difference between the plasma salicylate of those with rheumatoid arthritis and those with backache. 2 Two patients had plasma salicylate values that differed significantly from the remainder but neither these results nor the marginal differences between plasma salicylate levels of the others could be explained by individual variations in the capacity for excreting salicyluric acid or salicyl phenolic glucuronide. 3 Increasing the dose of aspirin in four patients demonstrated the reduced proportions of salicyluric acid and salicyl phenolic glucuronide excreted at high doses and the increased importance of unchanged salicylic acid as an excretory pathway. These findings are consistent with a limiting capacity for salicyluric acid and salicyl phenolic glucuronide synthesis and excretion. 4 The findings in one patient suggested that inter-subject variations in the capacity for producing salicyl phenolic glucuronide and salicyluric acid may have an effect on plasma salicylate levels at high doses of aspirin.