A drug-resistant Trypanosoma congolense strain with predetermined curative doses (CD50 and CD90) of samorin at 13.9 +/- 1.02 and 20.3 +/- 1.13 mg/kg body weight, respectively, was cyclically transmitted through tsetse flies and by syringe passages in mice in the absence of drug pressure. The changing levels of drug sensitivity were determined after every 3rd cyclic and 5th syringe passage intervals. It was noted that when the strain was maintained in tsetse flies through 12 cyclical transmissions, the CD50 and CD90 dropped slightly from 13.9 to 11.9 +/- 1.06 and from 20.3 to 18.0 +/- 1.08 mg/kg body weight, respectively. This decrease in the level of resistance was not significant (P greater than 0.05). However, when the trypanosomes were maintained by syringe passages in mice, there was a significant reduction (P less than 0.05) in the degree of resistance (CD50 from 13.9 to 11.4 +/- 1.07 and CD90 from 20.3 to 16.7 +/- 1.16 mg/kg), by the 15th syringe passage.