Abstract: The invention provides a method of monitoring the vasodilatory or vasoconstrictive effects of a physiologically active substance administered to a human or non-human animal body, said method comprising the steps of: administering said substance into said body; administering into the systemic vasculature of said body a contrast enhancing amount of an intravascular paramagnetic metal containing magnetic resonance imaging contrast agent; subjecting said body to a magnetic resonance imaging procedure capable of generating from magnetic resonance signals from said body a series of temporally spaced images of at least a part of said body into which said agent passes, said procedure being a fast imaging procedure having an image acquisition time of less than five seconds; and detecting temporal variations in said signals or images whereby to monitor the vasoconstriction or vasodilation induced by said substance.