Seasonal changes in the tolerance of osmotic stress in natterjack toads (Bufo calamita)

1. Bufo calamita were exposed to tap water to solutions of urea (up to 1000 mM) and NaCl (up to 250 mM). 2. The effects of season and nutrition on the adaptation to osmotic stress were studied by monitoring body mass, moulting rate, hematocrit, plasma osmolality and the plasma concentrations of urea, chloride and glucose. 3. There were marked seasonal changes in the time of survival in tap water as well as in the ability to acclimate to high concentrations of urea and NaCl. 4. The maximal concentration of urea in the plasma of starving toads was greater in winter (650 mM) than in summer (410 mM), but fed toads also reached winter levels (653 mM) during summer. 5. Salt acclimation was accompanied by urea accumulation in plasma up to 175 mM. 6. Salinities exceeding 454 mOsmol/l of NaCl were lethal.