In this preformulations study, rheological properties of microemulsions with progesterone (1%) were studied to analyze the effect of xanthan and chitosan at different concentrations (0.5-3%). Steady shear and oscillatory stress rheometer. Steady shear data were satisfactorily adjusted to the Carreau model. For all preparations, shear-thinning behaviour was observed. Zero shear viscosity (eta(0)) increased with the biopolymer concentration. The results from dynamic experiments showed the behaviour of all preparations with xanthan gum and those of chitosan to be characteristic of weak gets and liquid-like solutions, respectively. The correlation between dynamic and steady-shear properties (extended Cox-Merz rule) was satisfactory for the two polymers. The recovery analysis of micromolecules with xanthan showed a total recovery percentage of 90% for the highest concentrations of this polymer. However, microemulsions with chitosan showed practically no recovery. Progesterone release was greater for the microemulsions with chitosan. (c) 2008 Wiley Periodicals, Inc.