When ratio approaches are used to obtain reliable estimates for binding potential, the radioactivity must reach equilibrium during the acquisition time. The problem is that the time to reach equilibrium is highly dependent on the density of D2 receptors, which varies about 100-fold in the human brain. A consequence is that regions with low receptor density (i.e., the temporal cortex) will reach equilibrium long before regions with high receptor density (i.e., the striatum). As shown in simulations based on experimental data, the end-time method, a ratio method used in SPECT, overestimates the binding potential if the time to equilibrium is within the time of data acquisition but underestimates the binding potential if the time to equilibrium falls beyond the acquisition time (1). In SPECT studies using this method, low striatal and high extrastriatal binding potential have been reported (2, 3). In the reports of the PET studies referred to in the letter, the methods were not reported in detail, which makes it difficult to appraise the results (Meltzer et al., 1999; Xiberas et al., 1999). It cannot be discounted that these results were also influenced by the methods limitations of applying a ratio analysis on striatal binding at pre-equilibrium conditions.