Summary: In the present study, we investigated (a) the effects of the extract of Ruscus aculeatus, which is used to increase peripheral venous tone, on the diameter of arterioles (ID range 10–70 u.m) and venules (ID range 20–135 (μm) of hamster cheek pouch microvasculature in vivo and (b) the influence of temperature on the observed effects. For microcirculatory measurements, the preparations were placed under an intravital microscope and coupled to a closed-circuit TV (ccTV) system. The TV monitor display was used to obtain arteriolar and venular ID recordings (always at the same site) by an image shearing device. For systemic intravenous (i.v.) administration, the measurements were performed every 10 min, before (control) and after injection of the extract (5 mg/kg). During topical application, the extract was tested, in different concentrations, at 25°, 36.5°, and 40°C. Systemic i.v. administration of Ruscus extract evoked venular constriction and did not affect the arteriolar diameter or mean arterial pressure (MAP). Topical application of Ruscus extract elicited concentration- and temperature-dependent responses in the vessels. At 25°C, arterioles and venules dilated; at 36.5°C, the arterioles remained unchanged while the venules constricted, and at 40°C, the arterioles remained unchanged or constricted depending on the concentration used while the venules further constricted. The effects of Ruscus extract observed in vivo at the microcirculatory level further support the data previously reported on larger vessels and on patients with venous insufficiency.