The radioprotective action of dopamine on whole-body x-irradiated rats was investigated. Dopamine given immediately before exposure (700 to 1000 R) increased the number of survivors compared to irradiated controls. All irradiated controls died within 12 days after exposure. The spleen and testes of irradiated animals that survived by virtue of dopamine treatment showed a marked recovery. The weight and histologic structures of the spleen and testes of sham-irradiated animals receiving dopamine showed no change compared to normal controls. Dopamine in a single dose or in multiple doses after x-irradiation had no effect on percentage of survivors or survival time when compared to irradiated controls. L-Dopa had minimal radioprotective efficacy, although it had pharmacological effects similar to those of dopamine. Like dopamine, L-dopa was not effective after exposure.