An analysis was made of the effect of DDT and related substances on the membrane currents associated with step changes of the membrane potential. The substances tested are: DDT, DDD, DDE, DDMU, DDA, DDCN, bis‐(p‐chlorophenyl)‐acetamide and polychlormated bi‐phenyls (PCB). DDT. DDA and bis‐(p‐chlorophenyl)‐acetamide had acute effects on permeability parameters of the nerve membrane. The effects of the substances differed drastically from each other. DDT affected the turn off process of the sodium permeability mechanism after a positive potential step; the effect was large on fibres from Rana pipiens but negligible on fibres from Xenopus laeris. DDA affected the turn off process of the potassium permeability mechanism while bis‐“p‐chlorophenyl‘‐acetamide affected sodium and potassium permeability constants, Pxa and PK. The other substance; tested had no or only marginal acute effects. The main effect of DDA may be described as a specific effect on the rate constant βn. The obvious differences in effects of the substances on the permeability parameters were thus related to defined minute molecular differences of the studied substances.