A new water-soluble stimuli-responsive poly(N-acryloyl-N'-propylpiperazine) (PAcrNPP) was synthesized and characterized. The polymer exhibited a lower critical solution temperature (LCST) in water at 37 degreesC. The phase transition temperature was highly sensitive to pH changes. The effects of some simple salts and cationic surfactants were studied. The enthalpy of phase separation determined by microcalorimetry was 21.4 kJ mol(-1). This value corresponds to the breaking of one hydrogen bond per monomer unit. Dynamic light scattering studies indicated some aggregations of polymer chains below the LCST. No such aggregations were observed for the lower analogues, poly(N-acryloyl-N'-methylpiperazine) (PAcrNMP) and poly(N-acryloyl-N'-ethylpiperazine) (PAcrNEP) which exhibited no LCST. The cross-linked polymer gels were sensitive to both pH and temperature. Their response time to swelling and deswelling was 150 min. The water sorption of the gels was non-Fickian under both acidic and neutral conditions.