The PAP-technique and antibodies to myosin were used to demonstrate the prerequisites for vasoconstriction in the juxtaglomerular part of the preglomerular arteriole as compared with its proximal segment in rats and mice. In contrast with the myosin-positive/renin-negative proximal part of the afferent arteriole no myosin-like activity could be demonstrated in its distal, renin-positive part. In accordance, no thick myofilaments were found in fully differentiated juxtaglomerular epithelioid cells replete with mature secretory granules. Stimulation of the renin-angiotensin system was followed by an increase of the renin-positive/myosin-negative portions of the preglomerular arteriole. Marked interspecies and internephron variations in the length of this vessel segment under control and stimulated conditions were observed. The juxtaglomerular part of the preglomerular arteriole close to the macula densa seems therefore to have only limited capabilities for vasoconstriction. This finding may be of importance regarding the tubulo-glomerular feedback, a mechanism allegedly triggered by the so-called 'macula densa-signal'. It is suggested that this non-contractile segment of the afferent arteriole may represent the renal vascular receptor responsible for the increase of renin secretion during pressure reduction. Unlike the afferent arterioles, most of the efferent arterioles showed the highest level of their weak but distinct myosin-like immunoreactivity in the juxtaglomerular region, indicating some efferent juxtaglomerular vasoconstrictive ability.