The effect of three drugs on the receptor potential (ReP) of the crayfish retina measured with external electrodes was tested. 1. Tetrodotoxin (TTX) (1·10-6 g/ml and 1·10-5 g/ml) showed only slight effects on the receptor potential (Table I and II). 2. Veratridine (VERA) (1·10-5 g/ml) showed only small effects of the same type as TTX but a little stronger on the receptor potential which became obvious especially in the period after washing out the drug (Table III). 3. Tetraethylammonium (TEA) (5 mм/1) caused a decrease of the height of the receptor potential without influencing the repolarizing phase (decrease-time £2) markedly (Table IV). The results can be interpreted as follows: 1. The permeability increase for sodium, calcium and magnesium, whidi causes the rising phase of the receptor potential in the crayfish photoreceptor cell is of different nature than the increase of the sodium permeability in the squid nerve, or the sites of the sodium channel are not accessible for the TTX in the photoreceptor cell. This is shown by the experiments with TTX and VERA. 2. AU the changes of the receptor potential which are caused by TEA can be explained by assuming that the permeability for potassium in the dark is lowered by TEA and that in the course of the normal receptor potential an increase of potassium permeability, which causes the decreasing repolarizing phase of the receptor potential, does not occur.