The functional analysis of expressed human gene variants is important in the study of genetic susceptibility to diseases, pharmacogenetic traits and for the investigation of the human genetic diversity at the molecular level. We have performed the analysis of sequence polymorphisms in the human D5 dopamine receptor gene (DRD5) predicting missense and nonsense amino acid changes in the receptor protein. The amino acid substitutions in the human D5 dopamine receptor are: Leu88 to Phe in the putative second transmembrane domain, Ala269 to Val in the third intracellular and Pro330 to Gln in the third extracellular loops, Asn351 to Asp in the seventh transmembrane and Ser453 to Cys in the C-terminal domains and Cys335 to Stop in the third extracellular loop. The two amino acid substitutions in the transmembrane domains had an effect on agonist binding to the human D5 dopamine receptor. Asn351 to Asp resulted in an approximately 10-fold decrease in dopamine and threefold decrease in R(+)-SKF-38393 binding affinities. Leu88 to Phe resulted in a small increase in dopamine binding affinity. Antagonist binding affinities were mostly unaffected by the amino acid substitutions with the exception of Leu88 to Phe, which showed small reductions in binding affinities of SCH-23390 and risperidone. The existence of functionally different variants of the human dopamine receptors might have phenotypic consequences given their importance in central nervous system physiology and pharmacology.