Intron 9 of the human dopamine beta-hydroxylase (DBH) gene was amplified using a long PCR procedure in unrelated patients with schizophrenia and unrelated control subjects. MspI digestion of the PCR fragments showed a two allele polymorphism. A1 and A2 Kruskal-Wallis analysis revealed a significant difference in serum DBH activity among the three groups carrying the A1/A2 genotype, the drug-free and drug treated patients, and the control subjects (H = 12.2, df = 2, p < 0.005), and also a significant difference among the three subgroups of drug-treated patients carrying the genotype of A1/A1, A2/A2 or A1/A2 (H = 10.4, df = 2, p < 0.01). The present results suggest that the MspI polymorphic site in intron 9 of the human DBH gene may be associated with alterations of DBH activity in schizophrenia and with the influence of neuroleptic drugs on the DBH activity as well.