Diagram comparing dose-response curves for an agonist drug in different species with varying levels of spare receptors. A represents a receptor system requiring full receptor occupancy for full effect (the 10 circles represent all of the receptors [red = bound by agonist; white = unbound by agonist; X = bound by antagonist). R (rodent) and C (cat) represent receptor systems in which 50% and 20% occupancy, respectively, can achieve the maximal effect (ie, there are spare receptors). Conversely, in the presence of a competitive antagonist, similar receptor occupancy can lead to reduction of the effect in A but not R or C.