The insect stage of Trypanosoma brucei adapted the activities of 16 metabolic enzymes to growth rate and carbon source. Cells were grown in chemostats with glucose, rate limiting or in excess, or high concentrations of proline as carbon and energy sources. At each steady state, samples were collected for measurements of substrate and end product concentrations, cellular parameters, and enzyme activities. Correlation coefficients were calculated for all parameters and used to analyze the data set. Rates of substrate consumption and end product formation increased with increasing growth rate. Acetate and succinate were the major nonvolatile end products, but measurable quantities of alanine were also produced. More acetate than succinate was formed during growth on glucose, but growth on proline yielded an equimolar ratio. Growth rate barely affected the relative amounts of end products formed. The end products accounted for the glucose consumed during glucose-limited growth and growth at high rates on excess glucose. A discrepancy, indicating production of CO2, occurred during slow growth on excess glucose and, even more pronounced, in cells growing on proline. The activities of the metabolic enzymes varied by factors of 2 to 40. There was no single enzyme that correlated with consumption of substrate and/or end product formation in all cases. A group of enzymes whose activities rigorously covaried could also not be identified. These findings indicate that T. brucei adapted the activities of each of the metabolic enzymes studied separately. The results of this complex manner of adaptation were more or less constant ratios of the end products and a very efficient energy metabolism.