Promastigotes of Leishmania mexicana mexicana recently derived from amastigotes by transformation in vitro respired at a rate (17 nmol O2/min per 10(8) parasites) 4-5 times higher than that of amastigotes, but when the difference in cell protein content between the two preparations was taken into account the rates were not significantly different (32 nmol O2/min per mg protein). The respiration of both amastigotes and promastigotes was sensitive to cyanide, azide, antimycin A, 2-n-heptyl-4-hydroxyquinoline-N-oxide and high concentrations of amytal, but insensitive to rotenone and salicyl-hydroxamic acid, indicating that the two developmental forms possess a similar cytochrome-containing respiratory chain. D-Glucose and non-esterified fatty acids stimulated promastigote respiration and amastigote transformation to promastigotes in vitro; possibly these substances are important exogenous energy substrates for both forms of the parasites. Amino acids (incuding L-proline) and proteins did not appear to be used as energy substrates. The respiration rate of promastigotes was found to rise significantly upon continued sub-culture in vitro; at the same time cell size and protein content increased.