Subcellular distribution of chlormerodrin-203Hg in the rat kidney has been studied. Two hours after i.v. injection of the radiopharmaceutical, kidney tissue homogenate was submitted to differential centrifugation to obtain cell organelles. Radioactivity distribution in relation to total radioactivity of kidney homogenate obtained in nine repeated experiments was as it follows (+/- SD): nuclei 3.5% (+/- 0.5); mitochondria 6.7% (+/- 0.6); microsomes 9.3% (+/- 0.8) and cytosol 38% (+/- 3.9). Specific activities expressed as counts/min/mg of protein were 915 (+/- 13%), 1232 (+/- 14%) and 1097 (+/- 14%) for mitochondria, microsomes and cytosol, respectively. Specific activity of the nuclei was 1445 (+/- 16%) counts/min/mg DNA. These results demonstrate that this imaging agent penetrates into the kidney cells where it remains mostly in the cytosol. A fraction of chlormerodrin-203Hg binds to microsomes, mitochondria and nuclei.