The effects of ergot alkaloids and their derivatives on the excitability of an identifiable giant neurone, TAN (tonically autoactive neurone), found in the suboesophageal ganglia of the African giant snail (Achatina fulica Férussac) were examined. Ergometrine and methylergometrine had the strongest inhibitory effects. The critical concentrations were 3 X 10(-5) approximately 10(-4) kg/l. The three hydrogenated ergot derivatives, 9, 10-dihydroergocristine, 9, 10-dihydroergocryptine and 9, 10-dihydroergocornine, had weaker inhibitory effects than those of ergometrine and methylergometrine. The other substances examined, including D-lysergic acid, D-lysergic acid amide, D-lysergic acid piperidinamide, had no effect. Some of the substances examined, for example, lysergine, agroclavine, elymoclavine, nicergoline, ergotamine, bromocriptine and lergotrile, at high concentrations occasionally caused transient inhibition immediately after their application. This inhibition may be due to the synaptic influences produced by the excitation of other neurones. Some of the substances examined, lysergine, agroclavine, elymoclavine, festuclavine, nicergoline and lergotrile, at high concentrations caused abnormal spike discharges of TAN such as doublet or triplet spikes.