The absolute and relative bioavailability of three sustained release theophylline preparations marketed in Switzerland was tested in 11 healthy volunteers. The mean absolute bioavailability in the steady state in 4 to 6 volunteers was 88% (range 52-127%) for Euphyllin retard 350 mg, 93% (78-106%) for Phyllotemp retard 225 mg and 131% (78-169%) for Theolair SR 175 mg. There was no significant difference (p less than 0.05) in absolute or relative bioavailability between the three preparations. The concentration-time curves in a dosing interval showed adequate sustained release properties for all three preparations. The mean ratio between the concentration of theophylline in serum and saliva was surprising constant. Moreover, each volunteer appeared to have an individual fixed serum/saliva ratio. This ratio was independent of the theophylline preparation or the serum concentration. Knowledge of the individual serum/saliva ratio may allow calculation of the serum level from noninvasive tests in saliva during long-term treatment.