The effect of mono-and dichlorophenyl analogs of N-phenyl-2-methylpentanamide (PMP) and N-(3,4-dichlorophenyl)-alkylamides on the photolytic activity (Hill reaction) of chloroplasts isolated from turnip greens (Brassica spp.) was studied. Photochemical activity was measured potentiometrically with ferricyanide as the electron acceptor. PMP inhibited the Hill reaction by 50 percent (I50) at 4.5 × 10-5 M. Monochlorination in the meta and para ring positions enhanced the inhibitory activity whereas chlorination in the ortho position negated inhibitory activity of the parent chemical (PMP). N-(3,4-dichlorophenyl)-2-methylpentanamide (3,4-DCPMP) was the most inhibitory of the 6 dichlorinated isomers (I50 = 5.8 × 10-7 M). Replacement of the imino hydrogen of 3,4-DCPMP with a methyl group reduced inhibitory activity below that of the unsubstituted parent compound (PMP). Of the straight-chain N-(3,4-dichlorophenyl)alkylamides studied, the propanamide expressed maximum inhibition. 2-Methylalkylamides were more inhibitory than the corresponding straight-chain alkylamides. Maximum inhibition in the branched series was obtained with 3,4-DCPMP. Kinetic studies performed with 3,4-DCPMP at different light intensities indicated that both light and dark reactions were inhibited. However, the light reaction was more sensitive to the effects of the chemical.