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Aqueous solutions of Carbopol [polyacrylic acid (PAA)] are low viscosity acidic solutions that transform into gels upon an increase n the pH and, therefore, may be used as in situ gelling ophthalmic drug delivery systems. However, the amount of PAA required in the solution to form stiff gels upon instillation in the eye is not easily neutralized by the buffering action of tear fluid. A reduction in the PAA concentration without compromising the in situ gelling properties as well as the overall rheological behavior of the system can be achieved by adding a suitable viscosity‐enhancing polymer. The rheological properties of aqueous solutions containing PAA and hydroxypropyl methylcellulose (HPMC), a viscosity‐enhancing polymer, evaluated as a function of temperature and pH, were similar to those of pure PAA solutions; that is, both form low viscosity liquids at pH 4.0 and transform into stiff gels with plastic rheological …