We demonstrate a previously unknown regulation for intestinal-type alkaline phosphatase (IAP) as a heat shock protein (HSP). Heat shock to rat intestinal epithelial cells (IEC)-18 at 43°C induced the expression of IAP-I and HSP72 mRNAs time dependently (<60 min) but did not induce expression of IAP-II, tissue nonspecific-type alkaline phosphatase (TNAP), or HSP90 as determined by the RT-PCR method. To confirm the identity of the IAP-I gene, we sequenced the amplification product of IAP-I and found the gene to have 99% homology with the sequence of the IAP-I gene in rat intestine. Under the subculture conditions used, no IAP protein was detected in IEC-18 cells, but it became detectable as a 62-kDa band on a Western blot after heat shock. IAP-I was also induced by sodium arsenite, which generates reactive oxygen species and is an inducer of members of the HSP family. Glutathione suppressed activating protein-1 and cAMP response element-binding protein activation caused by heat shock but did not suppress the expression of IAP-I. These results suggest that cellular stress induces the elevation of IAP-I mRNA and protein synthesis. IAP-I may play an important role as a dephosphorylating enzyme under stress conditions.