Cholera toxin subunit B suppressed the proliferation of cultured vascular smooth muscle cells from the thoracic aorta of Wistar-Kyoto rats (WKY), stroke-resistant spontaneously hypertensive rats (SHR) and stroke-prone SHR (SHRSP). Since cholera toxin subunit B did not stimulate cyclic AMP accumulation in vascular smooth muscle cells, the effect of cholera toxin subunit B might be due to another mechanism. Cholera toxin subunit B bound to the surface of vascular smooth muscle cells and was rapidly incorporated into them. The morphological structure of vascular smooth muscle cells was transformed from the synthetic type to the non-synthetic type, in which microfilaments and intermediate filaments were abundantly formed, while rough endoplasmic reticulum was decreased after CTB treatment.