S-Adenosyl-L-methionine (SAMe) levels in liver tissue were reduced by 35% after isolation and washing of the organ. The apparent half-life of SAMe (1,75-25 microM) during liver perfusion was between 120 and 460 min; however the uptake of the labelled methyl group by hepatic tissue of fed rats was low (6%). This value increased to 12% in organs isolated from 24-hour fasted animals. Addition of SAMe to the perfusion medium increased tissue levels of ATP. Except for N-demethylation of aminopyrine and cytochrome c reductase, liver microsomal activity was not affected by treatment with SAMe either in vivo or/and in perfused liver.