Biomicroscopy was used to study order I-IV branches of the uterine artery in sexually mature female Wistar rats after a single 5- or 10-min direct exposure of the right broad ligament of the uterus to a helium-neon laser beam in the red band of the spectrum (λ=632.8 nm, output power 2 mW, intensity 20 mW/cm 2 ). Under the influence of laser exposure, there was a significant increase in the diameter of small uterine branches and a weaker increase in large vessels with a well-developed muscular membrane. Preliminary administration of zinc IX protoporphyrin hemoxygenase blocker (30 mmol/kg) reduced reactivity of the vascular bed to laser irradiation, mainly of order I-II branches. It was concluded that carbon monoxide can act as an intermediary between the laser exposure and uterine vessels.