The present study was undertaken to investigate the cerebral adaptation to hypoosmolar stress in adult Pannon white rabbits by applying proton nuclear magnetic resonance relaxometry. Progressive hyponatremia was induced by combined administration of hypotonic dextrose in water and 8-deamino-arginine vasopressin over a hydration period of 3, 24, and 48 h. Each group comprised five animals. After completing the hydration protocols, blood was taken to determine plasma osmolality (freezing point depression) and sodium concentration (ion-selective electrode) and, at about the same time, T2-weighted images were made. After the in vivo measurements, the animals were killed and brain tissue samples were obtained to measure water content (desiccation method) and T1 and T2 relaxation times (proton nuclear magnetic resonance method). Free and bound water fractions were calculated by using multicomponent fits of the T2 relaxation curves. It was shown that brain water content and T1 relaxation time remained unchanged despite the progressing hyponatremia. By contrast, T2 relaxation time increased steadily from the control value of 100.2 +/- 7.7 ms to attain its maximum of 107.5 +/- 8.5 ms (p < 0.05) after 48 h of hydration. Using biexponential analysis, fast and slow components of the T2 relaxation curve could be distinguished that corresponded to the bound (T21) and free (T22) water fractions. In response to hyponatremia, the bound water fraction was markedly depressed from 6.5 +/- 3.0% to 3.6 +/- 0.9% (3 h, p < 0.05) and 3.9 +/- 0.8% (24 h, p < 0.05); then it approached the initial value of 5.3 +/- 2.5% by the end of the hydration period of 48 h. It is concluded that restructuring of brain water is a contributory factor to the successful adaptation to hypotonic environment.