The effect of ethyl N,N-di-n-propylthiolcarbamate (EPTC) on photosynthesis was determined by C14O2 fixation by red kidney bean, Phaseolus vulgaris L., leaves following root application of this herbicide in a time-course study at various concentrations of the herbicide. EPTC inhibited C14O2 fixation at relatively high concentrations and long exposures of the herbicide, but did not alter the relative distribution of C14 in the various labeled compounds. The effect of EPTC on the respiration of excised embryos of corn and mung bean (Phaseolus aureus Roxb.) following 24 or 48 hr exposures to the herbicide. When the data were expressed on a fresh-weight basis, it indicated that EPTC stimulated respiration, but the same data expressed on an embryo basis did not show this stimulation. EPTC inhibited the growth and the concurrent increase in fresh weight of these embryos. The effect of EPTC on oxidative phosphorylation was determined with cucumber mitochondria. EPTC markedly inhibited both phosphorus uptake and oxygen consumption. Phosphorus esterification was more sentitive than oxygen utilization: this gave a reduced P/O ratio or an uncoupling effect. However, since the concentration of EPTC required to produce these effects was quite high, the physiological significance of the results is questionable.