G alpha z (Lounsbury et al. 1991) and G alpha 12 (Kozasa & Gilman, 1996) are excellent in vitro substrates for all three subtypes of protein kinase C (PKC). Activation of PKC in intact platelets by agents such as thrombin, thromboxane A2 (TXA2) analogues and phorbol esters leads to rapid and near-stoichiometric phosphorylation of G alpha z (Carlson et al. 1989). PKC can bind to G alpha z and facilitate its phosphorylation at Ser-27 (Lounsbury et al. 1993). This phosphorylation blocks the interaction of G alpha z with Gbeta:gamma suggesting that it is a regulatory mechanism for attenuating signalling by preventing subunit reassociation.