Investigations were carried out on the effects of several pesticides (Lindan, 2,4-D, p,p-Metoxychlor, Carbaryl, Monolinuron) on the transport ability of the plasma membrane. The experiments were done on isolated skin of the frogs Rana temporaria. The so called short-circuit current technique was used for quantitative evaluation of the transport activity in relation to sodium ions. The effect of the studied substances was studied during 3 hours after their addition to the medium on the external surface of frog skin after a preceding control period of 1-2 hours. A series of long-term experiments was performed also in which the animals were exposed to those pesticides for 14-20 days. The results of the acute experiments failed to demonstrate any significant effect of the studied substances on the plasma membrane, but some weak effects could have been observed in some experiments, at least. The results obtained in the long-term experiment showed that an effect on the plasma membrane could be postulated as regards 2,4-D and Carbaryl. It does not mean, however, that this effect is the only mechanism of the toxic action of these substances on the organism.