The relationship between serum and unstimulated and stimulated mixed saliva concentrations of nortriptyline was studied in seven healthy volunteers given a single oral dose of 50 mg. The concentrations were shown to be statistically significantly but variably correlated. The stimulation of saliva flow with Parafilm caused a significant increase in the mean +/- s.e.m. saliva pH from 6.93 +/- 0.15 to 7.34 +/- 0.09 (p less than 0.01). Simultaneously the mean saliva/mean concentration ratio decreased from 0.28 +/- 0.04 to 0.14 +/- 0.03 (p less than 0.001). This decrease correlated significantly with increases in flow rate and pH. Serum concentration could not, however, be reliably calculated from saliva concentrations because of the great variability in the saliva/serum ratios at any given pH.