Eukaryotic and prokaryotic cells undergo an increase in heat shock proteins, including hsp70, during exposure to environmental stress and during some developmental changes. In trypanosomatid protozoa such as Leishmania sp. that cycle between poikilothermic vectors and mammalian hosts, this heat shock response occurs at programmed times in the parasite's life cycle. The increase in heat shock proteins in mammalian cells is initiated by an increased rate of transcription, resulting in greater amounts of total hsp70 RNA and protein. In contrast, we found a dramatic increase in hsp70 RNA during growth of Leishmania chagasi promastigotes from logarithmic to stationary phase in liquid culture, which was not accompanied by an increased amount of hsp70 protein. Furthermore, there was a 1.8-fold increase in hsp70 protein induced by exposure of L. chagasi to superoxide, but this was not associated with an increase in hsp70 RNA. We conclude that in contrast to higher eukaryotes, the amount of hsp70 protein produced by Leishmania sp. is not regulated by the steady state level of total hsp70 RNA.