We investigated the permeabilities of lipid bilayers to the neurotoxin methylmercuric chloride (MMC). This mercurial is an efficient collisional quencher of the fluorescence of N-alkyl carbazole derivatives. Quenching of the fluorescence of beta-(3-(9-carbazole)-propionyl-L-alpha-phosphatidylcholine (CPA-PC) in vesicles of dimyristoyl phosphatidylcholine and of dioleoyl phosphatidylcholine reveal rapid diffusion of MMC in the alkyl side chain regions of these bilayers. By a combination of (1) the lipid concentration dependence of the apparent quenching constants, (2) the solubility of MMC in concentrated lipid dispersions and (3) the 270 MHz proton magnetic resonance of methylmercury in the presence of lipid bilayers we conclude that the lipid-water partition coefficient of this mercurial is less than or equal to two. Using the fluorescence quenching and the partitioning data we estimate the diffusion coefficient of MMC in these bilayers to range from 0.13 to 0.31 X 10(-5) cm2/sec, or 20--47% of its diffusion coefficient in ethanol. These data indicate that lipid bilayers do not pose a significant permeability barrier to the diffusional transport of MMC.