Smooth muscle cells were cultured from guinea-pig aorta and labelled with 45Ca++ and 32Pi to investigate the possible effect of cicletanine, a new antihypertensive drug, on the release of intracellular Ca++ and the metabolism of phosphoinositide induced by histamine. In 45Ca++ labelled cells, histamine increased in a dose-dependent manner the 45Ca++ efflux in the first two minutes. Stimulation of 45Ca++ release was observed with H1-agonist [2-pyridylethylamine dihydrochloride (2-PEA)] but not with H2-agonist (dimaprit). In addition, histamine- or 2-PEA- induced 45Ca++ efflux was inhibited by the H1-antagonists (mepyramine and terfenadine) whereas the H2-antagonist (cimetidine) was without effect. Similar results were obtained in 32Pi labelled cells; both H1-agonists (histamine and 2-PEA) increased the labelling of phosphoinositides. This effect was completely blocked by mepyramine. These results demonstrate that the histamine-induced stimulation of 45Ca++ efflux and phosphoinositide metabolism are mediated through H1-receptors. In the above systems, cicletanine was as effective as the H1-antagonist (mepyramine) with an IC50 of 10(-6) M for both 45Ca++ efflux and phosphoinositide metabolism. Blockade of these systems by cicletanine may be part of the mechanism by which this drug produces relaxation of blood vessels and may account for its in vivo antihypertensive action.