Drugs administered into the epidural space for selective spinal analgesia must diffuse through the spinal meninges to gain access to their sites of action in the spinal cord. Therefore, knowledge of the physical and chemical properties of drug molecules that govern their diffusion through the meninges is important for understanding the pharmacokinetics of epidural analgesia. To determine the physicochemical properties of drug molecules that govern the rate at which drugs diffuse through the spinal meninges, the authors measured the permeability coefficient of eight different drug molecules through the spinal meninges of the monkey using a previously established in vitro model. We previously reported permeability measurements for four of the molecules used in this study; the other four molecules' permeability measurements are new. The measured permeability coefficient was then correlated with the drugs' molecular weight, molecular surface area, molecular volume, length of the major molecular axis, and octanol:buffer distribution coefficient. We found no relationship between the drugs' permeability coefficients and any measure of drug mass, molecular shape, or molecular size. There was, however, a biphasic relationship between the octanol: buffer distribution coefficient and the drugs' measured permeability coefficients. Drugs that were either very hydrophilic or very hydrophobic had permeability coefficients that were significantly less than drugs of intermediate hydrophobicity. These data suggest that it should be possible to design novel analgesics for which meningeal permeability is maximal.