Two selective H2-histamine agonists, dimaprit and impromidine, have been tested for their action on histamine release from human basophils and rat mast cells. IgE-mediated basophil histamine release was inhibited by stimulation of histamine H2-receptors. However, differences between the actions of both dimaprit and impromidine were noticed. Both impromidine and dimaprit had no specific effect on 48/80-induced histamine release from rat mast cells, although the latter in higher concentrations either slightly increased spontaneous histamine release or non-specifically inhibited compound 48/80-induced release. The results are consistent with the view that activation of adenylate cyclase via H2-histamine receptors might be an important regulatory mechanism of histamine release from human basophils but not from rat mast cells.