1. MS-222 anaesthesia of the marine teleost Pagrus auratus caused a dose-dependent elevation in haematocrit. 2. At 60 mg/l MS-222 the elevated haematocrit was fully accounted for by erythrocyte swelling, but at 100 mg/l red cell count also increased. 3. Uptake of anaesthetic in all tissues was rapid and the rate of onset of anaesthesia was dose-dependent. 4. Stage III level of anaesthesia (loss of equilibrium) was correlated with critical levels of MS-222 in the blood and brain. 5. The presence of acetylated derivatives of MS-222 in the blood demonstrated degradation of the anaesthetic, and high levels in the liver and kidney suggested that these organs were the sites of drug metabolism.