Structural cooperative transitions in thermosensitive nonionic hydrogels of poly(N-vinylcaprolactam) (PVCa) were studied using high-sensitivity differential scanning calorimetry (HS-DSC). The thermoshrinking of the PVCa gel is endothermic, much like first-order phase transitions. According to the HS-DSC data, the gel system undergoes two successive cooperative transitions between 25 and 50 degrees C. The low-temperature transition (31.5 degrees C) is proposed to be associated with the microsegregation resulting in formation of hydrophobic domains (micromicelles) in the gel whereas the high-temperature transition (37.6 degrees C) is due to the gel volume collapse. The transition temperatures decrease in the presence of NaCl and increase with increasing sodium dodecyl sulfate (SDS) concentration. Dependences of the transition enthalpies and entropies on NaCl and SDS concentrations display a more complex character. The influence of sodium chloride and SDS on the transition temperatures is in agreement with the general expectations taking into account their effect on hydrophobic interactions and the ability to form micromicelles.