Ten male patients (mean age 37.3 +/- 6.4 years) with psychosis, intermittent hyponatremia, and polydipsia (PIP syndrome) underwent measurement of weight, sitting and standing blood pressure, and serum sodium concentration at 7 a.m. and 4 p.m. weekly for 8 consecutive weeks. Blood pressure was higher in the afternoon than in the morning. The diurnal decrease in serum sodium (141.4 +/- 2.8 to 134.2 +/- 4.8 mEq/l) was associated with a diurnal increase in weight (78.4 +/- 9.7 to 80.0 +/- 10.3 kg). When the weight increase was normalized by dividing by 7 a.m. weight (NDWG), the following relationship evolved: diurnal serum sodium decrease = 3.060 + [201.728 x NDWG]. Therefore, NDWG accounted for 63.1% of the variability of serum sodium. Using the known relationship of plasma water, total body water, and total body weight, we calculated that antidiuresis (afternoon weight gain) accounted for 62.5% of afternoon hyponatremia. Thus, two separate methods of calculating the relationship between antidiuresis and hyponatremia provided remarkably similar findings. We derived a table to predict 4 p.m. serum sodium values based on 7 a.m. weight, 7 a.m. serum sodium, and 4 p.m. weight.