Vasoactivity is an important physiological indicator of cardiovascular health which is frequently measured using ex vivo vessels to determine functional mechanisms and evaluate pharmacological responses. Currently, there are no imaging methods available to assess vasoactivity in multiple vascular beds of living animals noninvasively. In this work, we have developed methods to use photoacoustic tomography to assess vasoactivity in vivo in systemic vasculature of living animals. A spherical-view photoacoustic tomography system was used to monitor acute vasodilation in the whole abdomen of a pregnant mouse in response to injection of G-1. After 3D image reconstruction, the diameter of the iliac artery and photoacoustic signal intensity of a placenta over time was measured. The artery and placenta had differential response to the vasodilator G-1. We validated the observed vasodilation of artery by monitoring the change in cross-sectional diameter of an individual artery using standard B-mode ultrasound imaging.