As a microcapsule with temperature sensitivity, poly(methacrylic acid)-polyethylenimine complex capsules containing N-isopropylacrylamide units were designed. Two kinds of copolymers of methacrylic acid and N-isopropylacrylamide were synthesized by free-radical copolymerization. Partly crosslinked poly(methacrylic acid)-polyethylenimine complex capsules containing the methacrylic acid-N-isopropylacrylamide copolymers were prepared at 40 or 25 degrees C. The permeation of phenylethylene glycol through the capsule membranes was investigated. Permeability of the capsules prepared at 25 degrees C increased monotonously with increasing temperature from 10 to 50 degrees C. Permeability of the capsules prepared at 40 degrees C also increased with increasing temperature up to 25 degrees C but decreased above 30 degrees C. Also, the degree of swelling of the membranes prepared at 40 degrees C decreased above 30 degrees C. Differential scanning calorimetry measurement showed that N-isopropylacrylamide units underwent more efficient transition in the capsule membranes prepared at 40 degrees C than in the membranes prepared at 25 degrees C. The capsule membranes prepared at. 40 degrees C might have domains in which N-isopropylacrylamide units are concentrated, whereas these units should distribute uniformly in the capsule membranes made at 25 degrees C. Such a difference in distribution of N-isopropylacrylamide units might result in the different permeation property of the capsule membranes.