SUMMARY.— The rate of diffusion of methyl nicotinate in water, tetrahydrofurfuryl alcohol, propylene glycol and polyethylene glycol 300 has been investigated using optical density as a measure of the concentration of methyl nicotinate. The volatility and affinity for water, viscosities and surface tensions of these solvents have also been studied. The relationship between these physical properties and the influence of the solvent on the vasodilator response to methyl nicotinate is discussed. Copyright © 1969, Wiley Blackwell. All rights reserved