Quantitative estimation of neuroreceptor occupancy by exogenous drugs using positron emission tomography is based on the reduction in the total volume of distribution (V(T)) of site-specific radioligands after drug administration. An estimate of the distribution volume of free and nonspecifically bound radioligand (V(ND)) is also required to distinguish specific from total binding. However, a true reference region, devoid of specific binding, is often not available. We present a transformation of a graphical method, originally introduced by Lassen, using regional estimates of V(T) alone to determine occupancy, together with an extension that does not require baseline data.