The effect of ergot alkaloids, some of them from the ergosine series, was tested on the response of an identified neurone of Helix pomatia to the application of dopamine (DA) which was membrane hyperpolarization (DA-I response). To evaluate the antagonistic potency of the ergot alkaloids, the dose-response relationship of the response to DA was established first in the absence and later in the presence of the tested compound. The sigmoidal dose-response curves obtained were used to determine the pA2 values of all the antagonists tested. Comparing them it was found that among the ergosines tested, the most potent was dihydroergosine, its pA2 value being (6.27) similar to the values obtained for dihydroergotoxine (6.66) and dihydroergotamine (6.36). The two other unhydrogenated compounds (ergosine and saccharinoergosine) had potencies of the same order as bromoergocryptine. The D-isomer (+) of the saccharino compound was the least potent, but it acted as a DA-receptor antagonists, as well as the dextroisomer of bromoergocryptine, bromoergocryptinine. By comparing the current-voltage relationship of the neurone in the presence and absence of the ergot alkaloids tested it was found that they had no non-specific effects on the membrane. The ergot alkaloids did not change the amplitude of depolarization of the neurone induced by gamma-aminobutyric acid (GABA) and acetylcholine (ACh).