Isolated pig heart pyruvate dehydrogenase complex (PDC) has been reported to have a molecular mass of 8000 kDa (large PDC) and a diameter of about 45 nm. Studies were carried out to determine the size of PDC in situ. Active enzyme centrifugation showed that extracts of pig heart mitochondria contain, in addition to large (S20,w = 100-200 S) active complexes, catalytically active small PDC (S20,w = 30 S). In addition, small PDC (1000-3000 kDa) could be obtained by gel filtration of mitochondrial extract. If pure large PDC was chromatographed in Triton X-100, then a fraction of it appears in the 1000-3000-kDa range. Isolation of small PDC and rechromatography showed the formation of large PDC. Anti-PDC and ferritin-labeled second antibody were used in an attempt to determine the size of PDC in isolated inner membrane vesicles containing PDC and in permeabilized mitochondria. In both studies no large aggregates of ferritin particles were found which would correspond to the size of large PDC. The conclusion of these experiments is that PDC exists in situ in a smaller form than the isolated pure enzyme.