INTRODUCTION The regulation of cellular growth and division by extracellular signals is now a major field of investigation. Within this area of study considerable attention has focused upon the intracellular signalling pathways which initiate early events associated with mitogenesis. These include systems in which specific kinases are activated following the generation of second messengers, for example diacylglycerol (DAG)/protein kinase C (PKC) and the cyclic AMP/cyclic AMP-dependent protein kinase (PKA) path-ways.

Recently, major advances have been achieved in the elucidation of cellular signalling pathways which involve the activation of proteins or enzymes by phosphorylation upon tyrosine residues. In this type of pathway cellular signals are generated through a series of protein-protein interactions and a cascade of phosphorylation events rather than by the immediate generation ofa second messenger. Historically this type of pathway has been associated with growth factor receptors which display intrinsic tyrosine kinase activity [1]; however, a number of G-proteincoupled receptors have also been found to activate tyrosine kinase pathways. Concentrating on early signalling events, this review will outline the regulation and function of the major tyrosine kinase pathways activated in response to both classes of agonist and place these in the context of other well-established cell signalling pathways.