The concentration of tetracycline in gingival fluid was measured following the oral administration of single doses of 250 or 500 mg. Six volunteers received a single dose of either 250 mg or 500 mg of tetracycline and gingival fluid was sampled at 15-min intervals during the first 2 h, 30-min intervals for the following 2 h and at hours 5, 6 and 7. Four volunteers were given a single dose of either 250 mg or 500 mg and were sampled every hour for 24 h. Gingival fluid was sampled using an intracrevicular technique from four gingival sites and blood was obtained by finger puncture. The concentration of tetracycline in gingival fluid peaked at 3 1/2-7 h achieving average levels typically in the range of 5-12 micrograms/ml. Blood levels peaked at 3-4 h and reached values between 1.0 to 2.6 micrograms/ml. Tetracycline was detectable in gingival fluid at least 19 h after a single dose but rarely was detectable at 24 h. The results demonstrated that tetracycline in the gingival fluid was typically 2-10 times blood levels after a single dose.