['Hyaluronan (HA) has a major role in regulating synovial fluid volume. This role depends on the synovium functioning as an ultrafilter that reflects HA during trans-synovial fluid drainage. Reflection boosts the HA concentration on the membrane surface, leading to osmotic retention of synovial fluid ("buffering"). In arthritic effusions, however, HA concentration and osmotic buffering are greatly reduced. We tested the hypothesis that reflection is reduced (escape increased) when the HA concentration falls below the molecular entanglement concentration (C*).', 'HA at 0.2 mg/ml (<C*) or 1.5 mg/ml (>C*) was infused continuously into rabbit knee joints to set up a steady trans-synovial filtration. Joint-derived lymph was sampled over 3 hours, and subsynovial fluid was sampled at the end of the 3-hour period. HA was quantified by high-performance liquid chromatography to evaluate the reflected fraction. C* was determined by viscometry.', 'Viscometry showed that 0.2 mg/ml HA was below C* and 1.5 mg/ml was above it. At 0.2 mg/ml, the mean +/- SEM HA reflected fraction was 0.66 +/- 0.04 (n = 7). At 1.5 mg/ml the reflection increased to 0.88 +/- 0.04 (n = 5) (P < 0.005). HA permeation increased almost 3-fold, from 12% to 34%, at the lower concentration.'] Is hyaluronan molecular reflection by synovial lining concentration dependent and reduced in dilute effusions in a rabbit model?