Rat liver nucleotide pyrophosphatase/phosphodiesterase I (NPP/PDE) catalysed efficiently the transfer of adenylate from ATP to alcohols (methanol, ethanol, propanol, ethylene glycol, glycerol, 2, 2-dichloroethanol and glycerol 2-phosphate), which acted as adenylate acceptors competing with water with different efficiencies. NPP/PDE kinetics in alcohol/water mixtures were accounted for by rate equations for competitive substrates, modified to include alcohol negative co-operativity and, depending on the nature of the alcohol, enzyme denaturation by high alcohol concentrations or activation by low alcohol concentrations. The correlation of alcohol efficiencies with alcohol acidities, the comparison of rat liver with snake venom NPP/PDE, and the different effects of ionic additives on the efficiencies of glycerol 2-phosphate and glycerol provided evidence for interaction of the alcohols with a base catalyst, a non-polar and a cationic subsite in the active centre of rat liver NPP/PDE. The enzyme thus appears to be well suited to act as transferase, and we propose that NPP/PDE could be an adenylylating agent in the membrane.