The solubility of ammonia, alphaNH3 (mM/mmHg), was determined at 37 degrees C and low ammonia partial pressure (0.02-1 mmHg) in pure water (n =24) as 46.70+/-0.40; aqueous isotonic salt solutions (n = 7) as 46.8+/-0.81; and human plasma (n = 5) as 42.0+/-0.66. The last figure increases to 45.3+/-0.63 if expressed in molal units (mmol/kg plasma water x mmHg) instead of molarity with respect to the water content of the plasma (mean from four healthy and fasting donors: 0.908+0.005 kg H2O/kg plasma; mean density at 37 degrees C: 1.020+/-0.002 kg/l). In pure water, the solubility value is the mean of three different methods: (a) extrapolation of the salting-out effect of ammonia in aqueous NaOH to zero concentration; (b) slope of Henry-Dalton's law and (c) directly measured in pure water and 0.001 M aqueous NaOH. Based on the Henderson-Hasselbalch equation for the system NH4/NH3 in isotonic salt solutions and human plasma, both constants, apparent pK and solubility, can be derived from total ammonia concentration and pH at equilibrium with defined ammonia gas phase, if additionally the concentration of NH4 or NH3 is known. This was verified, in the first case, by determining the concentration of NH4+ by the experimental conditions, and in the second, by two measurements of total ammonia concentration at two different pH values. Total ammonia concentration was measured by a specific enzymatic standard test and pH with the glass electrode. The mean apparent pK was 8.968+/-0.013 in isotonic salt solutions (n = 7), and in human plasma (n = 10) it was 9.014+/-0.033.