A three-parameter equation which successfully quantifies the turnover time in goldfish exposed to variable concentrations of eight alkyl esters of p-aminobenzoic acid is described. One parameter characterizes the target concentration necessary to induce narcosis, and the other two parameters represent the resistances encountered by the drug when it migrates through aqueous and lipid phases from source to target. The octanol-water partition coefficients, or aqueous solubilities, and melting points of the esters are included to account for variation in drug hydrophobicity, the relationship between these properties being quantified. The inactivity of high-melting-point esters is explained by the constraint imposed by melting point on the maximum aqueous concentration which can be achieved by solid solutes.