Further mapping of the Achatina giant neurone types sensitive to the neuroactive peptides isolated from invertebrates. 1. The effects of the 10 synthetic neuroactive peptides originally isolated from invertebrates, applied locally to the neurone tested by the brief pneumatic pressure ejection on the identifiable neurone types of Achatina fulica Ferussac were examined. 2. Achatin-1 (Gly-D-Phe-Ala-Asp), an Achatina endogenous tetrapeptide having a D-phenylalanine residue, ejected locally, showed the depolarizing effects on nearly half of the number of neurone types tested. 3. ACEP-1 (Ser-Gly-Gln-Ser-Trp-Arg-Pro-Gln-Gly-Arg-Phe-NH2), isolated originally from Achatina atria, and pedal peptide (Pro-Leu-Asp-Ser-Val-Tyr-Gly-Thr-His-Gly-Met-Ser-Gly-Phe-Ala) and buccalin (Gly-Met-Asp-Ser-Leu-Ala-Phe-Ser-Gly-Gly-Leu-NH2), found in Aplysia neurones, showed excitatory effects on some Achatina neurone types. 4. Myomodulin (Pro-Met-Ser-Met-Leu-Arg-Leu-NH2), found in Aplysia neurones, produced a hyperpolarization on nearly half of the number of Achatina neurone types tested. The two FMRFamide-like peptides, <EDPFLRFamide (<Glu-Asp-Pro-Phe-Leu-Arg-Phe-NH2), isolated from Helix heart, and AF1 (Lys-Asn-Glu-Phe-Ile-Arg-Phe-NH2), from Ascaris head, also showed hyperpolarizing effects on more than half of the number of Achatina neurone types. 5. SALMFamide 1 (Gly-Phe-Asn-Ser-Ala-Leu-Met-Phe-NH2), isolated from Asterias nervous system, CCAP (Pro-Phe-Cys-Asn-Ala-Phe-Thr-Gly-Cys-NH2), from Carcinus pericardial organ, and corazonin (<Glu-Thr-Phe-Gln-Tyr-Ser-Arg-Gly-Trp-Thr-Asn-NH2), from Periplaneta cardiac corpus, had no effect on Achatina neurones.