Oxamniquine (OXY), a tetrahydroquinoline derivative, is used as an antischistosomal drug and generally has been labeled with carbon-14 and tritium. We decided instead to label it with technetium-99 (99mTc). In order to determine the optimal conditions, different concentrations of this drug were incubated with various stannous chloride solutions. We then added 99mTc, and chromatography was performed using 0.9% NaCl solution, acetone and 1.2N HCl as the mobile phase. Using a solution of 1.0 mg/mL stannous chloride and 0.5 mg/mL oxamniquine, over 94% of the radioactivity bound to oxamniquine (99mTc-OXY). In the biodistribution study, 99mTc-OXY was administered in mice intramuscularly, orally and intravenously. When the intramuscular route was used, the main uptake (after 30 minutes) of the labeled drug was in the kidneys, liver and intestines; after 240 minutes the labeled drug was still found in the liver and kidneys, but at increased levels in the intestines. It was also present in the faeces. When the oral route was employed, labeled OXY was mainly found in the stomach after 30 minutes, but there was a decrease after 240 minutes. During this period radioactivity increased in the intestines. When the intravenous route was employed the labeled OXY was found in the liver and spleen. The radioactivity decreased with time in these organs. Using infected animals, radioactivity was found in isolated worms.