The therapeutic concentrations of antipsychotic drugs in the patient's plasma water or spinal fluid are identical to their blocking potencies in vitro at the dopamine D2 receptor, with the exception of clozapine which acts at D4. The variation in K values between laboratories stems from the fact that the apparent K value for any antipsychotic drug depends on the affinity of the competing radioligand for the receptor or the membranes. Clozapine at the D2 receptor has a K value of 420 nM using [3H]nemonapride, 180 nM using [3H]spiperone and 82 nM using [3H]raclopride. These K values are related to the tissue/buffer partition coefficients of the ligands. Extrapolating down to either 1 or 0 partition yields the intrinsic K values for the antipsychotic in the absence of any competing ligand. The extrapolated or intrinsic K value for clozapine at D4 is 1.3 nM, in agreement with the value of 1.1 nM measured directly with [3H]clozapine at D4. Clozapine in vivo, however, must compete with endogenous dopamine in the synapse, estimated as 50 nM. Thus, the in vivo concentration of clozapine for 75% occupation of dopamine D4 receptors can be derived as approximately 14 nM, in agreement with the observed value of 12-20 nM in the plasma water or spinal fluid in treated patients. In L-DOPA psychosis in Parkinson's disease, the clozapine concentration (in the plasma water or spinal fluid) for 75% blockade of dopamine D4 receptors may be predicted as approximately 3 nM, in general agreement with the value of approximately 1.2 nM in Parkinson patients who have L-DOPA psychosis. These considerations provide strong support for the conclusion that clozapine primarily _targets the D4 receptor in psychosis. Using the same considerations for haloperidol, it can be shown that the haloperidol therapeutic concentration required for 75% blockade of dopamine D2 receptors in vivo will be approximately 2-3 nM, in agreement with the observed value in the spinal fluid or plasma water of 1-3 nM.