In Rat-9G cells several copies of the major immediate early (IE) transcription unit (regions 1 and 2) of the human cytomegalovirus (HCMV) are stably integrated. The cells show a repressed phenotype for IE expression but can be induced by inhibition of protein synthesis. In this report we present evidence that the repressed phenotype is due to the absence of IE transcription and that heat-shock and sodium arsenite treatments each result in the transcriptional activation of the repressed IE transcription unit. Either treatment resulted in the induction of HCMV IE transcripts and IE nuclear antigen expression. An octameric DNA sequence present in three of the 18 bp IE enhancer elements (GGACTTTC) resembles the cellular heat-shock element core consensus sequence and may therefore be involved in the heat-shock response.