Activities of segments of the electron transport pathway of mitochondria isolated from pollen of Typha latifolia L. during the course of germination in vitro were compared with those of mitochondria in intact grains. For this purpose, suitable inhibitors and artificial substrates were selected for their ability to penetrate through the exine, intine, and plasmalemma. In contrast to their counterparts in vivo, mitochondria isolated during the initial stages of germination exhibited low rates of electron transport, resulting from loss of NAD+ and displacement of cytochrome c from its site of action. The phosphorylative capacity was also impaired. Great caution must be exercised therefore, before interpreting results obtained with isolated mitochondria.