Two short-term, double-blind, multi-centre studies, one in rheumatoid arthritis and the other in osteoarthritis, were carried out to investigate the efficacy and tolerance of two formulations of the new osmotic delivery system containing sodium indomethacin trihydrate ('Osmosin') compared with conventional indomethacin capsules and placebo. Both formulations contained the equivalent of 85 mg indomethacin. 'Osmosin' was designed to deliver drug in solution at a constant rate of 7 mg per hour, the other formulation at 9 mg per hour. The results indicated that 'Osmosin' administered once or twice daily was at least as effective in reducing disease symptoms as 25 mg indomethacin capsules 3-times daily. In addition, the combined incidence of gastro-intestinal side-effects reported in the two studies was significantly lower with 'Osmosin' than with the other active drug groups. The possible contribution of this novel drug delivery system towards patient compliance as a result of less frequent administration and fewer digestive system side-effects is discussed.