1. The effects of a range of RFamide peptides were examined on the frequency, amplitude and tone of the isolated heart of Achatina fulica. 2. FMRFamide, FLRFamide and SDPNFLRFamide were potent excitants while LPLRFamide, KHEYLRFamide and GSFFRFamide were weak excitants. 3. DNFLRFamide and SSLFRFamide were potent inhibitory peptides while LFRFamide, GSLFRFamide and KNEFIRFamide were weak inhibitory peptides. 4. MRFamide, LRFamide and RFamide were inactive. 5. It is concluded that a tetrapeptide sequence is the minimal requirement for activity on the Achatina heart. With the exceptions of DNFLRFamide and in most preparations GSFFRFamide, LRFamide peptides are excitatory while FRFamide peptides are inhibitory. 6. Due to its inhibitory effect and high potency, the sequence DNFLRFamide warrants further investigation.