The enzyme spermidine/spermine N1-acetyltransferase (N1-SAT) is rapidly induced by heat shock in CHO and A549 cells, with activity declining by 24 h. Depletion of intracellular polyamines by alpha-difluoromethylornithine, an inhibitor of ornithine decarboxylase, blocks this induction. Re-addition of putrescine to these cultures restores the response to heat shock, with a concomitant increase in intracellular N1-acetylspermidine. Diaminopropane is more than twice as effective as the naturally occurring diamine putrescine, suggesting that the propylamine moiety of spermidine is involved in the regulation of N1-SAT induction. Inhibitor studies indicate transcriptional activation and that the enzyme has an apparent half-life of 30-60 min. A second heat shock rapidly inhibits induced N1-SAT activity, which decays with a half-life of 2-3 min. Despite its induction by heat, N1-SAT is not a stable enzyme, suggesting that the activity observed is not due to a modification of an existing peptide, but is due to a transcriptional event, which may justify the inclusion of this enzyme in the family of heat-shock proteins.