Alterations in the expression and activity of guanine nucleotide regulatory proteins (G proteins) have been linked to the growth of several human tumors. We hypothesized that the expression and activity of G proteins are altered in human hepatocellular carcinoma (HCC). The expression of Gi and Gs proteins was determined in six human tumors and six normal controls (adjacent nonneoplastic liver) by Western blotting using specific antisera raised against the α subunit of G proteins Gi1, Gi1-2, Gi3, and Gs. Differences in G-protein expression were quantified by densitometry and expressed as percentage change from normal controls. The expression of Gi α1 was significantly increased in 80% of tumors (Gi α1, 284% ± 77%; P < .05 percent of normal tissue), whereas Gi α1-2 and Gi α3 expression was increased in 67% of tumors (Gi α1-2, 218% ± 21%; Gi α3, 154% ± 6%; P < .05 percent of normal tissue). The functional activity of Giα proteins as determined by pertussis toxin-catalyzed adenosine diphosphate (ADP)-ribosylation was also significantly increased in these tumors. In contrast, Gs α-protein expression was significantly reduced in all tumors examined (74% ± 8% of normal tissue, P < .05). The functional activity of Gs α, as determined by adenylyl cyclase (AC) activity, was significantly decreased in tumor as compared to normal liver under both basal and agonist stimulated (guanosine triphosphate gamma S and forskolin) conditions. In summary, these data show for the first time a significant alteration in G-protein expression and functional activity in human HCC tissue. These alterations indicate a down-regulation of the AC-linked enzyme effector system in HCC that may be of critical importance to the formation and progression of human hepatocellular carcinoma.