The binding of barbiturates to serum proteins impedes the diffusion of the drugs from the intravascular compartment. In certain concentration ratios sulphafurazole* considerably impedes the albumin binding of thiopentone and increases the intensity of the hypnotic effect of barbiturates in rats. In a blind trial on forty-eight patients the previous administration of sulphafurazole reduced the hypnotic, anaesthetic and total dose of thiopentone, and shortened the time to awakening. The number of undesirable side effects of the thiopentone and reactions to painful stimulation during anaesthesia were significantly reduced by sulphafurazole. The effect of sulphafurazole on thiopentone anaesthesia may be explained by the competitive dislocation of thiopentone from its binding sites on plasma proteins, thus increasing the concentration of thiopentone available for diffusion.