Angiotensin-converting enzyme (ACE) modulates dopamine turnover in the brain and catechol-O-methyltransferase (COMT) enzyme is an important agent in the metabolic inactivation of dopamine and norepinephrine. Functional polymorphism in the COMT and ACE genes causes variation in enzyme activities. We investigated the relationship of COMT and ACE gene polymorphism with response to conventional neuroleptic treatment in schizophrenia. In this study population we had earlier detected that COMT genotype is associated with unsatisfactory drug response. A total of 94 schizophrenic patients were evaluated either as responders (n=43) or non-responders (n=51). The responders had experienced a fair and steady response to conventional neuroleptics. The non-responders had failed to achieve an acceptable response to conventional neuroleptics. We also used a control population of 94 age- and gender-matched blood donors. Genotyping of the COMT and ACE genes was performed by polymerase chain reaction. The risk of having both low activity COMT and high activity ACE genotypes was over 10 times higher (odds ratio=10.89, 95%CI 1.14-103.98, P=0.04) in the non-responders compared to responders. ACE genotype alone did not differ between any groups. This finding may suggest a possible interaction with low activity COMT and high activity ACE genotype in association with poor response to conventional neuroleptics.