The method of fluorescence energy transfer is used to measure the distances between several sites on aspartate transcarbamylase. Both fluorescence steady-state and lifetime techniques are used. When the tryptophans on the catalytic subunit are the fluorescent donor groups, either pyridoxamine phosphate, covalently bound to an amino group at the active site, or 8-anilino-1-naphthalenesulfonate, noncovalently bound at the active site, is the acceptor group. The distance between tryptophan and the active site is calculated to be 2e A assuming that the fluorescence of only one tryptophan per catalytic polypeptide chain is quenched by the acceptor or 27 A assuming that both tryptophans on a catalytic chain are equally quenched. The pyridoxamine phosphate label is also used as the fluorescent donor with mercurinitrophenol bound to the sulfhydryl group of the catalytic subunit as the energy acceptor. For this pair of labels the active site is determined to be very close to the sulfhydryl group on the same catalytic chain and 26 A from the sulfhydryl groups on the other chains of the catalytic trimer. In experiments with pyridoxamine phosphate at the active site as the donor and 8-anilino-1-naphthalenesulfonate at the active site as the acceptor, a distance of 26 A between active sites of a catalytic trimer is found. No energy transfer is observed from pyridoxamine phosphate at the active site to a fluorescamine derivative of cytidine 5'-triphosphate at the regulatory site. This implies that these groups are separated by at least 42 A in the native enzyme. All of the distances are calculated using the assumption of rapid rotation of donor and acceptor dipole moments relative to the donor fluorescence lifetime. Fluorescence polarization measurements suggest this assumption does not produce a significant error in the calculated distances. The distances between the various sites are related to the subunit structure of aspartate transcarbamylase.