Circadian variation has been demonstrated in several clinical parameters used to assess disease activity in rheumatoid arthritis, but circadian variation in modern laboratory assessments has not been studied in depth. We therefore made 2-hourly measurements of plasma viscosity, C-reactive protein, total serum sulphydryl, and serum histidine on samples obtained over a 24-hour period from 6 patients with classical or definite rheumatoid arthritis. Hourly control samples were also taken from 6 normal volunteers, 3 of whom starved from 2200 h the previous night and 3 of whom ate normally. There was no significant variation in any of these laboratory measurements between 0900 and 1800 h either in patients or controls. These findings enable us to eliminate circadian variation as a source of error when using these laboratory tests in clinical trials of slow-acting anti-rheumatoid drugs.