Leucocyte migration was studied in vivo using a skin window technique, and in vitro by migration under agarose. No difference was found between 28 patients with rheumatoid arthritis (RA), 10 patients with psoriatic arthritis (PA) and 30 healthy controls. Most patients were under treatment with anti-rheumatic drugs. Patients treated with non-steroidal anti-inflammatory drugs (NSAIDs) had significantly lower values (p less than 0.01) than untreated patients. In vivo but not in vitro migration decreased during short-term treatment with diclofenac and naproxen, an effect observed both in patients and in healthy individuals. After pre-incubation of normal polymorphonuclear leucocytes with diclofenac, in vitro migration was diminished only at concentrations of 50 micrograms/ml and above, which are at least 10 times higher than those attained clinically. The in vivo effect of NSAIDs on leucocyte migration may imply a long-term disease modifying influence in chronic arthritides.