The adenylyl cyclase signal transduction system, a ubiquitous second messenger system, has been identified as a potential marker for genetic risk of alcohol and drug dependence. Using the polymerase chain reaction (PCR) to amplify exon 13 of the Gsα gene, two alleles were distinguished by denaturing gradient gel electrophoresis. One allele, designed A, contained the previously published C in the codon for asparagine 371, while the second allele, designated A, contains a C-T transition that conserves the asparagine residue at codon 371. The neutral polymorphism eliminates a Fok I restriction enzyme cleavage site, allowing use of restriction fragment length polymorphisms of PCR products to determine allelic frequency in 235 subjects with alcohol and/or drug dependence and in 85 control subjects. Since allele frequencies differ significantly by race, comparisons between affected individuals and controls were conducted separately for white and black groups. Within race, there were no significant differences in the frequency of the A allele among alcoholics, subjects dependent on cocaine or opioids, subjects dependent on these drugs and alcohol, and controls. We conclude that there is no association between alcohol and/or drug dependence and alleles of an exon 13 polymorphism of the Gsα gene in either black or white individuals.