The binding of many polypeptide hormones to cell surface receptors does not appear to follow the law of mass action. While steady-state binding data are consistent in many cases with either heterogeneous populations of binding sites or interactions of the type known as negative cooperativity, study of the kinetics of dissociation of the type known as negative cooperativity, study of the kinetics of dissociation of the hormone receptor complex allows an unambiguous demonstration of cooperative interactions. Negative cooperativity, which seems to be wide-spread among hormone receptors, provides exquisite sensitivity of the cell at low hormone concentrations while buffering against acutely elevated hormone levels. The molecular mechanisms underlying the cooperativity are still largely unknown. Cooperativity may stem from a conformational transition in individual receptors or involve receptor aggregation in the fluid membrane (clustering) or more extensive membrane phenomena. Thus, new models of hormone action must be considered which integrate the progress in our knowledge of both the complex mechanisms regulating hormone binding to their surface receptors, and the dynamic properties of the cell membrane.