The stably BK virus (BKV)-transformed hamster cell line BKT-1B and control BHK-21 cells were treated with dibutyryl cAMP (bu2cAMP), the adenylate cyclase activator forskolin, and the tumour-promoting agent 12-O-tetradecanoylphorbol-13-acetate (TPA). Cultures were stimulated for 30 min (short) and for 24 h (long). Northern blot analysis showed that for bu2cAMP and TPA both short and long stimulation resulted in significant increases in the levels of BKV early transcripts. Short exposure to forskolin resulted in a moderate increase and long exposure in a definite increase. In all cases the increased levels were maintained for at least 24 h after short stimulation was stopped. Experiments including the transcription inhibitor actinomycin D revealed that the enhanced levels of early BKV expression after treatment with the stimuli were due to induced RNA synthesis rather than to stabilization of the RNA. No DNA amplification of the early BKV sequences could be detected in the induced cells. The results are discussed with regard to possible roles for a cAMP-responsive element and a phorbol ester-responsive element, shown by sequencing to be present in the control region of the integrated BKV genome of the BKT-1B cells.