A major goal of molecular pharmacology is to account for the extreme biological potency and great diversity of action of hormones. These extracellular signals circulate in the blood at nanomolar or subnanomolar concentrations, and yet within seconds they can produce millimolar changes in the levels of metabolites. Hormonal stimuli must therefore be amplified a million-fold or more following their interaction with plasma-membrane-bound rcceptors. Furthermore, the actions of a hormone are nearly always pleiotropic, affecting many processes within a givcn target cell, and eliciting quite distinct responses in diffcrent cell types. Amplification and diversity in hormone action are achieved by two principal mechanisms, namely the reversible phosphorylation of proteins (Fig. 1) and the formation of molecular species termed ‘second messengers’. Many key regulatory proteins exist in either a phosphorylated or a dephosphorylated form, their steady-state levels of phosphorylation reflecting the relative activities of the protein kinases and phosphatases that catalyse the interconversion process. Phosphorylation of seryl, threonyl, or occasionally tyrosyl residues, trigger small conformational changes in these proteins, which alter their biological properties. Hormones transmit information to the interior of the cell by activating transmembrane signalling systems that control production of a relatively small number of chemical mediators or second