The changes of P300 parameters (lower amplitude and increased latency) are thought to be the most prominent phenomena of schizophrenia. A role of gene polymorphism in P300 generation was supported by several associative studies in psychiatrically well subjects and patients with mental disorders. We studied P300 parameters and the following polymorphisms: T102C for the serotonin receptor type 2A (5-HTR2A) gene, the 5-HTTLPR for the serotonin transporter gene, -809G/A, -616G/C N -52C/T SNPs in the promoter region of the dopamine D4 receptor (DRD4) gene and the Val158Met polymorphism of the catechol-O-methyltransferase (COMT) in 74 patients with schizophrenia and spectrum disorders and 71 their first-degree relatives. No association was found between serotonergic system genes and P300. The -809G/A DRD4 gene polymorphism was related to amplitude in all frontal leads (p=0,01) in patients. In relatives, an association was observed between -521C/T DRD4 variants and latency (p=0,005) as well as between the COMT gene polymorphism and P300 amplitude (p=0,004) at the central lead. Thus, the genes involved in dopaminergic system play a role in P300 generation both in patients with schizophrenia and spectrum disorders and their relatives.