The solubility of cyclosporin A was determined in water and aqueous solutions of a surface active poly (oxyethylene)-poly(oxypropylene)-poly(oxyethylene) block copolymer (poloxamer 188 or pluronic R F68 (PF68)) at concentrations and temperatures ranging from 4 to 22 g/l and from 10 to 50°C, respectively. The solubility behaviour was different between PF68 solutions and water. Solubility values indicated an exothermic heat of solution in each case except for PF68 solutions above 10 g/l at 37-50°C, where a change to endothermic heat of solution was detected. The slopes corresponding to water samples or 4g/l PF68 solutions at temperatures above 37°C do not statistically differ from zero. Aqueous solutions of the poloxamer 188 were also studied using frequency spectrum analysis (dynamic light scattering) and size exclusion chromatography. Below 40°C and 16 g/l essentially invariant values for the hydrodynamic radius were found with broad polydispersities associated. Increasing temperature and poloxamer concentration, the hydrodynamic radius also increased and the systems showed a narrower size distribution, possibly due to micelle formation according to the closed association model.