Adult Wistar rats injected with the dopamine receptor agonist apomorphine display different types of motility patterns with respect to oral stereotypes and locomotor activities. It was tested whether phenotypes exhibiting either 'sniffing' or 'non-sniffing' behaviour differed in gene structures of dopamine receptors D1 or D2. Forty-five Wistar rats of both genders were tested after a single dose of apomorphine (2 mg/kg s.c.) for stereotyped behaviour. Sequence analysis of the 5' flanking region, the 5' untranslated region and the coding region of the two genes revealed a new sequence for the 5' flanking region of the D1 receptor gene and two polymorphisms in the promoter region of the D2 receptor gene. The D2 receptor polymorphisms were denominated as T-355C- and A-257G-polymorphism. Between 'sniffing' and 'non-sniffing' animals no differences were detected in their D1 receptor sequence but there was a significant (P<0.05) association of the two phenotype groups with the D2 receptor genotype. These findings indicate that in Wistar rats at least part of an individual apomorphine stimulated behavioural phenotype might be due to an individual dopamine receptor D2 genotype.