Serum concentrations and urinary excretion of proxyphylline have been measured in five healthy adults after intravenous (29 mumol/kg), single oral (21 mumol/kg) and multiple oral (21 mumol/kg three times a day) doses to produce steady state. The mean peak time after oral administration was 29 min. The mean fraction absorbed was 1.09 calculated from serum concentrations, and 1.05 calculated from urinary excretion of the drug. The apparent volume of distribution was 0.61 1/kg (0.53--0.72 1/kg), 26% higher in males than in females. A two-compartment open model was found to describe the decline in the serum concentrations, giving a mean distribution half-life of 6 min. The intersubject ranges of biological half-life were 8.1--12.1 h and 8.3--12.6 h calculated from serum and urine data, respectively. 24% (18--29%) of the dose was excreted unchanged in urine, which agreed with the relationship between the calculated total body clearance and the renal clearance of the drug.