Using cell-free extracts ofNitrobacter agilis, we investigated the effects of CIPC and eptam on the oxidative phosphorylation coupled to nitrite oxidation as well as upon the reduced nicotinamide adenine dinucleotide (NADH2) oxidation to determine possible effects of the herbicides upon electron transport. Both CIPC and eptam exerted an uncoupling effect on the oxidative phosphorylation linked to nitrite oxidation. Although nitrite oxidation was inhibited, an intensely severe inhibition of phosphorylation caused a 50% reduction in the P/O ratio (μmoles of32P esterified per μatom of oxygen consumed) at about 2.3 × 10−4 MCIPC and at 8 × 10−4 Meptam. Likewise, 2,4-dinitrophenol (DNP), a classical phosphorylation uncoupler, affected the oxidative phosphorylation inN.agilis. Although oxygen uptake was inhibited, a 45% reduction in the P/O ratio occurred at 2.5 × 10−4 MDNP because of a greater effect on phosphate esterification. Neither CIPC nor eptam affected reduced nicotinamide adenine dinucleotide (NADH2) oxidase activity.