Inhibition of Hill reaction by organophosphorus insecticides was examined to clarify their mechanism of phytotoxic action on Chinese cabbage leaves. Among organophosphates tested, those which caused no phytotoxic symptom did not affect Hill reaction at concentrations below 10^-4M. Degree of inhibition of Hill reaction varied dependent on the organophosphates in the case of those causing necrotic symptoms on Chinese cabbage leaves. Dimethoate did not affect Hill reaction, while fenthion and fenitrothion inhibited reaction by 50% at a concentration of 10^-5M. Organophosphates causing chlorotic symptoms were strong inhibitors and suppressed the reaction at 10^-5-10^-6M. All the inhibitors were hydrophobic. Even in the case of a series of compounds belonging to straight-chain alcohols, fatty acids, and polychlorobenzenes, the increase in their molecular weight and concomitant decrease in water solubility apparently were in parallel with their ability to increase inhibitory activity. Thus, hydrophobic compounds are considered to be inhibitory for Hill reaction. These compounds may have high affinity to chloroplasts resulting in inhibition of the electron transport in them and causing decomposition of chlorophyll.