Paramagnetic substances that shorten proton T1 relaxation time have promise as nuclear magnetic resonance (NMR) contrast agents. The cardiovascular toxicity and tissue T1 response to 0.01-0.1 mM/kg intravenous doses of MnCl2 in rabbits and dogs were studied. There were dose-dependent changes in cardiac electrophysiology and arterial blood pressure consistent with the calcium-blocking properties of manganous ion. There were also dose-dependent and organ-specific changes in tissue proton T1. The sensitivity of organ T1 was expressed as the dose of manganese required to shorten the normal T1 by 50%. Liver, kidney, and heart were most sensitive, while skeletal muscle and adipose tissue were insensitive. MnCl2 is an effective NMR contrast agent, but its toxicity may be excessive for clinical use.