We analysed a model assuming that the G protein activation process is an enzyme catalysed reaction: (i) GGDP recognition, (ii) GDP release, (iii) GTP binding and (iv) activated GGTP (or betagamma followed by alphaGTP) release. This model suggests (1) that the agonists Kact value may decrease with increasing G protein expression. At low G protein concentrations, the agonists Kact value reflects binding to the 'low affinity state'. If the G protein concentration is saturating, the agonists Kact values are correlated but not identical to their KD values for the 'high affinity state' observed in binding studies. (2) All the G proteins recognized by a receptor behave as competitive antagonists with respect to each other. (3) Overexpression of G proteins which recognize only the active receptor state may result in constitutive receptor activation. The rate of G protein activation can be accelerated by facilitating G protein recognition, by increasing the rate of GDP release, or by activating G protein release. Our model explains why guanyl nucleotides are essential for G proteins activation yet inhibit agonist binding, and why the ability of receptor ligands to simulate G protein activation is not always correlated with their ability to distinguish two receptor states, at equilibrium. It can also explain the inhibitory effects of GDP and of the G protein betagamma subunit.