Human glycogen synthase kinase-3 (GSK-3) is a multisubstrate, proline-directed kinase that phosphorylates tau, beta-amyloid and neurofilaments. In this study, the expression levels of the two GSK-3 isoforms, alpha and beta, RNA and proteins in different human tissues were examined. Northern analysis demonstrated that GSK-3alpha is encoded by a 2.6-kb mRNA and GSK-3beta by 8.3- and 2.8-kb mRNAs. The two GSK-3beta mRNA species were variably expressed in different tissues. Northern and quantitative polymerase chain reaction demonstrated that both GSK-3alpha and GSK-3beta mRNA were prominently expressed in testis, thymus, prostate and ovary but were low in adult lung and kidney. Western blot analysis showed that the 51-kDa GSK-3alpha protein was highly expressed in lung, ovary, kidney and testis, whereas the 46-kDa GSK-3beta protein was highly expressed in lung, kidney and brain. The differential expression of GSK-3alpha and GSK-3beta mRNA and proteins and the lack of relationship between transcription and translation in some tissues indicate that GSK-3alpha and GSK-3beta are subject to different means of regulation.