We investigated the role of amino acids, by site-directed mutagenesis, in five of the seven transmembrane regions of the human dopamine D1 receptor. The results demonstrate a role for an aspartic acid (Asp70) in transmembrane 2 mediating the sodium ion effect on receptor conformation. Amino acids residues in transmembrane 3 and 5 are important for optimum dopamine binding to the receptor. Mutant receptors involving amino acids in transmembrane 7 suggest that binding sites for agonists and antagonists have distinct binding determinants within the dopamine D1 receptor.