To assess the importance of the cysteine residues Cys347 and Cys351 in the carboxylic tail in the human D1 dopamine receptor, seven mutant receptors were constructed by PCR. The pharmacological and functional properties of the wild-type and mutant receptors were assessed following transient expression in COS-7 cells. Affinities for [3H]SCH 23390 of mutant S347 (Cys347-->Gly), T348 (Tyr348-->stop), S351 (Cys351-->Gly), T351 (Cys351-->stop), T352 (Pro352-->stop), and S347/S351 (Cys347-->Gly and Cys351-->Gly) were similar to that of wild-type receptor, whereas the expression levels were reduced up to 80%. The potency of dopaminergic antagonists for these mutant receptors was very similar to that of the wild-type receptor. However, mutant T347 (Cys347-->stop) showed a 15-25-fold reduced affinity for the antagonists SCH 23390, (+)-butaclamol, and cis-flupentixol, thus not allowing radioligand analysis. Wild-type and mutant receptors responded dose-dependently with similar potency to dopamine and SKF 38393 with an increased adenylyl cyclase activity. However, mutant receptors with the Cys347 residue changed or removed displayed a diminished ability to activate adenylyl cyclase. Dopamine preexposure desensitized wild-type and mutant S351 receptors. However, mutant receptors with Cys347 replaced or the distal part of the carboxyl tail removed were unable to desensitize. Thus, Cys347 in the cytoplasmic tail of the human D1 dopamine receptor is important for the receptor in maintaining the conformation for antagonist binding, to play a crucial role in activation of adenylyl cyclase, and to be essential for agonist-induced desensitization.