Convincing data support the hypothesis that genetic factors are involved in the etiology of attention-deficit hyperactivity disorder (ADHD). Various lines of evidence have shown that the dopamine system plays a crucial role in the pathophysiology of ADHD. The dopamine D3 receptor gene (DRD3) represents a promising candidate to examine in ADHD. Animal studies have shown that DRD3 mRNA is highly expressed in the ventral striatum suggesting an involvement of this receptor in the control of motor behaviour. Manipulation of DRD3 in rodents has led to a mouse model with nonfunctional D3 receptors that displays hyperactive behaviour in various environmental conditions. Furthermore, administration of 7-OH-DPAT, a dopaminergic agonist that binds preferentially to D3 receptors exerts an inhibitory effect on locomotor activity while D3 antagonists induce hyperactivity. Among various polymorphisms described for DRD3, the BalI polymorphism is most interesting because it codes for an aminoacid substitution in the N-terminus of the receptor. The receptor products of the two alleles (Ser/Gly) exhibit differential affinity for dopamine. To determine if DRD3 Ser9/Gly is involved in the susceptibility to ADHD we genotyped 39 adults with ADHD and their respective parents (trios). Adult ADHD represents a promising phenotype for studying the genetic component of the disorder. In fact, a recent family study has shown that relatives of adult ADHD patients have a higher rate of ADHD compared to relatives of children with ADHD suggesting a stronger genetic component for the adult version. The results of genotyping in the 39 trios analyzed with the transmission disequilibrium test showed no excess of transmission for DRD3 MscI/BalI alleles (chi(2) = 0.360; df = 1; P = 0.54). This result, although from a relatively small sample, indicates that it is unlikely that DRD3 is playing a major role in the etiology of ADHD in our sample.