99mPertechnetate (99TcO-4) was injected intravenously in 7 healthy subjects and 30 patients with rheumatoid arthritis (RA). The uptake of isotope (99Tc) in both knee joint regions was traced continuously for 30 min from the time of injection. The isotope showed more rapid accumulation with increasing arthritic involvement of the knee, and the rate of uptake of 99TcO-4 in the knee region tallied with the clinical findings before and after treatment (14 RA patients). The findings indicate that the rate of 99TcO-4 uptake could serve as a useful complement to the more commonly used quantitative uptake of isotope. The two measures may possibly reflect different features of synovial inflammation.