AbstractEarlier studies on human essential hypertension, as well as on SHR, suggest that an increased media thickness, partly encroaching upon the vascular lumen, forms the main background of the increased flow resistance in primary hypertension (e.g. Folkowet al. 1970). In the present investigation a comparison was made concerning the distensibility of the resistance vessels in order to further explore the proposed difference in vascular design. The simultaneously perfused hindquarter vessels of SHR and NCR were exposed to sudden, repeated shifts of pressure and flow, first at maximal vasodilatation and then at stable levels of smooth muscle tone, induced by barium ions or noradrenaline. Relative changes of vascular internal radius (ri) were calculated from the pressure‐flow values, assuming rito be proportional to4flow conductance. For given changes in distending pressure (Pd), the SHR resistance vessels exhibited considerably smaller percentual richanges than the NCR ones, whether starting from the same Pd, the same ri, or even when both Pd and riwere initially kept identical in NCR and SHR by exposing the SHR vessels to lower concentrations of constrictor agents. This evidence of reduced vascular distensibility in SHR was obvious both at maximal dilation and, particularly, when smooth muscle tone was increased. This strongly supports the view that especially the media component of the SHR resistance vessels is enhanced in bulk, thus markedly influencing both “active” and “passive” resistanc