A library of "hydraphile" synthetic ion channel analogues that differ in overall length from ∼28-58 Å has been prepared. A new and convenient ion-selective electrode (ISE) method was used to assay Na+ release. Liposomes were formed from three different phospholipids: 1,2-dimyristoleoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dioleoyl-sn-glycero- 3-phosphocholine (DOPC), and 1,2-dierucoyl-sn-glycero-3-phosphocholine (DEPC). The acyl chains of the lipids comprise cis-unsaturated 14:1, 18:1, or 22:1 residues, respectively. Sodium release was measured for each liposome system with each of the synthetic channels. Peak activity was observed for shorter channels in liposomes formed from DMPC and for longer channels in DEPC. A separate study was then conducted in DMPC liposomes in the presence of the putative membrane-thickening agents cholesterol and decane. Peak activity was clearly shifted to longer channel lengths upon addition of 20 or 40 mol % cholesterol or n-decane to the liposome preparation.