The reduction of pertechnetate-99 by excess stannous chloride in several media was investigated as a possible model for radiopharmaceutical preparations which are based on the reduction of pertechnetate-99 m by the same reagent. Polarographic and idiometric techniques were used to analyze for unreacted stannous ion. It was found that pertechnetate is reduced to the +4 state in a citrate buffer at pH 7, to the +3 state in a DTPA buffer at pH4, and possibly to the +4 state in HCl. The reaction in DTPA solution was found to be complete within several minutes. The reduction in the citrate buffer proceeded rapidly to the +5 state, and much more slowly to the +4 state. The implications of these rate studies for the radiopharmaceutical preparations were discussed. Direct potentiometric titrations of pertechnetate-99 with stannous chloride showed that the technetium was in the +4 state in HCl, in the +5 state in citrate solution, and in both the +3 and +4 states in DTPA.