Macroporous hydrogels are characterized by large pore sizes, high pore volumes, and high specific surface area. Besides these characteristics, macroporous hydrogels based on thermally reversible polymers respond to temperature changes much faster than hydrogels prepared by a conventional method. Crosslinked poly(N-isopropylacrylamide) (polyNIPAAm) forms a thermally reversible hydrogel which shows a lower critical solution temperature (LCST) ca. 33°C in aqueous solutions. We have synthesized thermally reversible polyNIPAAm hydrogels having macroporous structures by a new method. These macroporous hydrogels have large pore volumes, large average pore sizes, and faster macromolecule permeation rates in comparison to conventional polyNIPAAm hydrogels synthesized by a conventional method. Compared with conventional polyNIPAAm hydrogels, the macroporous polyNIPAAm hydrogels have higher swelling ratios at temperatures below the LCST and exhibit faster deswelling and reswelling rates. The deswelling rates are especially rapid. These thermally reversible macroporous hydrogels may be very useful in controlled active agent delivery and toxin removal, as well as dewatering of solutions. Peptides or proteins may behave as if they were in bulk solution within the large aqueous pores, and this may reduce their inactivation when such gels are used for their storage and later release. The gels may also be useful in microrobotic devices due to their fast response to temperature. © 1992 John Wiley & Sons, Inc.