Alterations imposed by herbicides on the membrane potential (Δψ), oxygen utilization, and ATP synthesis of intact mung bean mitochondria were measured under state 3 conditions. Effects were correlated with changes imposed by classical electron transport inhibitors, energy transfer inhibitors, and uncouplers. In the dose-response studies, complete inhibition of ATP synthesis produced by electron transport inhibitors (rotenone, antimycin A, KCN), uncouplers [bis(hexafluoroacetonyl)acetone (1799) and carbonyl cyanide 4-trifluoromethoxyphenylhydrazone (FCCP)], and the herbicides was associated with a decrease in Δψ from the state 3 value of 126 mV to between 90 and 100 mV. In contrast, the complete inhibition of phosphorylation produced by the energy transfer inhibitor N,N′-dicyclohexylcarbodimide correlated with an increase in Δψ from the state 3 to the state 4 potential (145 mV). In the titrations, the herbicides and classical uncouplers, but not the electron transport inhibitors, progressively collapsed Δψ below the potential associated with the complete inhibition of phosphorylation (to the apparent Donnan potential of 60 mV). The herbicides could be placed into two groups according to the dose-response relationships exhibited with respect to Δψ and oxygen utilization. The first group, designated as dinoseb types (dinitrophenols, benzimidazoles, benzonitriles, thiadiazoles, and bromofenoxim), uncoupled phosphorylation and collapsed Δψ to the Donnan level before oxygen utilization was inhibited. These compounds possess dissociable protons and are postulated to act as protonophores, much like 1799 and FCCP. With the second group, termed dicryl types (acylanilides, dinitroanilines, diphenylethers, bis-carbamates, and perfluidone), collapse of Δψ was paralleled by uncoupling of phosphorylation and inhibition of oxygen utilization. However, phosphorylation was inhibited to a greater extent than was respiration. The dicryl-type herbicides are not classical-type protonophores. Some of their action can be attributed to interference with the redox pumps. The complete collapse of Δψ to the Donnan potential is associated with alterations and perturbations induced in the membranes by classical uncouplers and by both types of herbicides. The perturbations are postulated to increase the permeability of the membranes to protons and other cations and to induce unfavorable conformational changes that impede interactions between redox enzymes. Conceivably, the combined responses collapse Δψ and inhibit electron transport.