The differential permeability of cell membranes to various electrolytes and non-electrolytes represents one of the most fundamental characteristics of life. In the context of parasitology, differences in transport properties when parasite and host cells are compared represent an important avenue for chemotherapy. For example, a recent success in this area exploits the difference in nucleoside transport systems between host and parasite to produce combination therapy of Schistosoma japonicum by tubercidin and nitrobenzylthioinosine-5′-monophosphate (Elkouni, Diop & Cha, 1983). The morning session of this symposium was devoted to various aspects of membrane structure and function, particularly in the context of characterizing parasite transport systems. It is therefore probably worth while to present a simple classification of membrane transport systems, based on their complexity, and energy requirements. Table 1 summarizes the principal types of transport to be considered, starting with simple (Stokesian) diffusion. This will depend on permeant size, charge and lipid solubility. Classically, transmembrane transport of such molecules as aliphatic alcohols, ureas and small sugars has been studied, in simple systems such as the red cell (see Stein (1986) for references). In fact this route is important for intracellular delivery of a variety of hydrophobic molecules, for example phenylalanine benzyl ester transport as an antisickling agent (Acquaye, Young, Ellory, Grecki & Wilchek, 1983), or quin-2 and fura-2 for intracellular calcium measurements (Tsien, 1981).