SUMMARYOne hundred and forty-three healthy adult patients were given thiopentone until the eyelid reflex was abolished. The dose given correlated better with lean body mass (LBM) than with surface area or body weight. The mean dose of thiopentone was 3.95±0.58 mg/kg LBM in the single-observer group of 84 patients and 4.01±0.72 mg/kg LBM in the blind-study group of 59 patients. In patients of similar weight LBM was a better guide than body weight in estimating the induction dose of thiopentone. LBM is better suited for predicting induction dosage because of the tissue distribution time sequence of thiopentone. LBM has a much larger blood flow per mass of tissue than does fat.