The lifetime of a ligand-receptor complex within the endosomal compartment may be an important factor influencing the types of response produced by a particular receptor. Insulin, which has a relatively short endosomal residence, elicits primarily acute metabolic effects, whereas EGF and IGF-I, which have longer endosomal residences, elicit mitogenic responses. These differences in biological responses could reflect differences in activation of intracellular signalling molecules at specific cellular locations. Thus differences in ligand residency, along with differences in the intrinsic activity of receptors, could contribute to different downstream responses.