A model of drug action is proposed which is based upon intramolecular rearrangement of the drug-receptor complex. The model is a modification of classical occupancy theory kinetics. It is able to account for the transient excitatory response that is evoked by some blocking agents; it can also account for the phenomenon of “fade” (Paton, 1961) whereby the tissue response to a spasmogen exhibits an initial upstroke in its time-course, followed by decay to a plateau. The proposed kinetic model is discussed in relation to current structural models of adrenergic and cholinergic receptors.