Equations for the binding models are based on the law of mass action and conservation of mass. Two mass action equations (c)-(d) for dissociation constants derived from (a)-(b) and three conservation of mass equations (e)-(g) formed the five equations solved simultaneously for the two sites modeltotal for saturation binding. Four mass action equations for dissociation constants derived from (i)-(l) and four conservation of mass equations (m)-(p) formed the eight equations solved simultaneously for the two sites modeltotal for homologous or heterologous competition. L was the independent variable for two sites modelfree for saturation binding, which did not include Eq. (g). B was the independent variable of two sites modelfree for competition, which did not include Eq. (p). One site modeltotal excluded terms referring to the second site. The two sites model for saturation binding that ignored ligand depletion was based on Eqs. (c)-(f) and assumed LT = L. Notation: αL = constant describing NSB of radioligand; Kd1 = dissociation constant of high affinity binding site; L = free radioligand; LT = total radioligand; NSBL = nonspecific binding of radioligand; R1 = unbound first binding site; R1L = radioligand bound to first site; R1T = total high affinity binding site; B = free competitor (blocker). With analogous notations, the index "2" in these equations refers to the low affinity binding site.