The long terminal repeat (LTR) of human immunodeficiency virus type 1 (HIV-1) is activated under different conditions including heat shock. By using transient transfection assays, we have compared the thermal activation of HIV-1 LTR to that of the promoter of the gene encoding the human stress protein hsp70 which is under the control of the heat shock transcription factor HSF. In these assays, the chloramphenicol acetyl transferase (Cat) gene was used as a reporter gene. Several parameters of the heat stress were analyzed such as the temperature, the duration of heat stress and that of the recovery period. Under every condition tested, we have found that the kinetics of activation of both promoters were very similar. In addition, both showed a similar inhibition by actinomycin D. These results were compared to those obtained with a DNA construct containing the early promoter of SV-40 virus coupled to the Cat gene. In this case, no heat-mediated accumulation of CAT protein was observed, indicating that the transcriptional activation of HIV-1 LTR by heat shock is specific. HIV-1 LTR contains two NF-kappa B binding elements, involved in the activation of this promoter during oxidative stress, which are sequence related to the heat shock element HSE. However, under all the heat shock conditions tested, we have been unable to detect the binding of any protein to kappa B elements, suggesting that this site is not directly involved in the thermal activation of HIV-1 LTR. These results indicate that the thermal transcriptional activation of HIV-1 LTR and hsp70 promoters occurs through different mechanisms that are triggered by similar heat shock conditions.