After the injection of [3H]acetyl-labelled hyaluronan into normal rabbit knee joints, about 90% of its isotope content was ultimately accounted for as 3H2O. The rate of elimination of hyaluronan from synovial fluid was therefore estimated from changes in the level of 3H2O in plasma. The half-life of plasma 3H2O was 6.2 days (S.D. 0.7). As estimated from its metabolism to 3H2O, the mean intrasynovial half-life of [3H]hyaluronan of high molecular weight (modal relative molecular mass (Mr) greater than 6.0 x 10(6) was 13.2 h (range 11-15.5 h; n = 4); an otherwise identical preparation of low molecular weight (modal Mr 0.09 x 10(6] exhibited a mean half-life of 10.2 h (range 7.8-13.5 h; n = 4). The difference between the two groups was significant (P = 0.029). Both estimates were nevertheless close to those determined by others in the same species for injected proteoglycans (Mr 2.5 x 10(6), t1/2 = 12 h) and for endogenous hyaluronan calculated from changes in concentration during intravenous infusion of fluid under anaesthesia (t1/2 = 16 h). The similarity suggests that hyaluronan and proteoglycan are removed from synovial fluid by a common pathway with limited dependence on their molecular dimensions and concentrations.