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1. 5-Hydroxytryptamine (5-HT) has been iontophoretically applied to the membrane of central neurones of Cryptomphallus aspersa; CILDA neurones (cells with inhibition of long duration) (Gerschenfeld & Tauc, 1964) are the only cells sensitive to 5-HT. The responses to 5-HT is always a depolarization. The CILDA cells studied were also depolarized by ACh.

2. From experiments in which pulses of 5-HT and ACh were applied from a double-barrelled micropipette to the CILDA cell soma, it has been calculated that 5-HT and ACh receptors were located at different distances from the injecting micropipette tip. It has also been calculated from the diffusion equation that in the same CILDA cell a 5-HT concentration of 8·2 x 10^-9 M and a ACh concentration of 1·3 x 10^-8 M caused a similar peak depolarization.

3. CILDA neurones show `anomalous' rectification. 5-HT increases the membrane conductance of CILDA.

4. 5-HT receptors of CILDA neurone are desensitized by repeated application of 5-HT. The desensitization lasts for ca. 40 sec.

5. 5-HT receptors are blocked by lysergic acid diethylamide and its derivatives. Morphine chlorhydrate blocks them non-competitively.

6. Some inhibitors of monoamine oxidase (trancylpromine, isocarboxazide, iproniazide and nialamide) have been tested. They do not prolong the action of 5-HT, but block the 5-HT receptors.

7. No crossed desensitization between 5-HT and ACh has been observed. Atropine blocks both ACh-receptors and 5-HT receptors, 5-HT receptors appear to be blocked to a greater extent.

8. The data presented support the assumption of a excitatory transmitter role of 5-HT to CILDA neurones, but further evidence is necessary to confirm this hypothesis.

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