Chitosan-poly(acrylic acid) (CS-PAA) nanoparticles, to be used as ophthalmic drug carrier, were successfully prepared using template polymerization of acrylic acid (AA) in a chitosan solution. When the polymerization was done at 70 degrees C for 45 min with a CS/AA weight ratio of 1:1, the surface structure of the prepared nanoparticles was most stable with the smallest mean diameter (92.0 +/- 7.5 nm) and a stable zeta potential (25.5 +/- 2.6 mV) in a buffer solution (pH 4.5). The size of the nanoparticles dramatically increased with the pH value of the medium. Both in vitro and in vivo studies revealed that the prepared nanoparticle suspension was better at sustaining the release of pilocarpine than either simulated tear fluid or commercial eye drops.