The present study constitutes an effort to close the gap still existing between in vivo and in vitro experiments in vascular mechanics. Active and passive mechanical properties of the rat tail artery were studied in situ under conditions as close as possible to the natural in vivo state of the vessel. At constant pressure levels, changes in diameter as caused after dilatation by papaverine and constriction by norepinephrine, were automatically and continuously registered using a contact free measurement technique (Video Dimension Analyzer). In order to check consistency of the in situ findings with analogous results from in vitro studies, pressure-diameter and stress-strain data were compared with results from our own and other authors. The results show that, quite similarly to in vitro, the maximum isobaric response of the tail artery in situ occurs in the physiological pressure range of some 100 mm Hg. Although the results focus the attention on the importance of both the experimental protocols and the definitions used, they confirm the validity of in vitro investigations as a powerful tool in arterial rheology.