The inhibitory effect of methylmercury on rat liver mitochondrial D 3-hydroxybutyrate deshydrogenase--an enzyme of the inner membrane matrix, which requires lecithin as a cofactor and has thiol residues in the active site--has been investigated. Using a partially purified enzymatic extract, methylmercury inhibition of the reactivation of the apodeshydrogenase by liposomes of lecithin has been studied as a function of lecithin concentration in the incubation medium. Partial reactivation has been observed at a concentration 3 times higher than that needed to reactivate the control. The present studies support the hypothesis that phospholipids are implicated in the mechanism of methylmercury inhibition.