Incubation of Leishmania tarentolae promastigotes in 1.0 microgram/ml Berenil for 96 hr resulted in approximately 33% inhibition of cell growth and approximately 42.5% dyskinetoplasty in the cell population. The buoyant density of kinetoplast DNA (kDNA), p = 1.703 g/ml, remained unchanged after 96-hr exposure to the drug. Endogenous respiration as well as proline- and glucose-induced respiration dropped markedly after 36-hr exposure to Berenil. This drop occurred approximately 12 hr before the onset of dyskinetoplasty, a result which suggests that this drug adversely affects mitochondrial respiratory activity of the promastigotes.