The apparent biologic half‐lives of tetracycline, demethylchlortetracycline, methacycline, and doxycycline were found to increase from 6.3, 11, 7, and 8.3 hours to 10, 14.7, 11, and 14.5 hours, respectively, during 4 days of repetitive dosing (every 12 hours). Steady‐state serum levels predicted mathematically on the basis of the latter biologic half‐lives agreed with those observed experimentally, whereas steady‐state serum levels predicted on the basis of the biologic half‐life calculated from single‐dose studies did not. A method was developed for estimating the biologic half‐life from steady‐state serum level data; these estimates agreed well with the biologic half‐life estimated from semilogarithmic plots of serum level versus time data during the steady state and after dosing had ceased. Capsules of tetracycline phosphate complex, tablets of demethylchlortetracycline hydrochloride, capsules of methacycline hydrochloride, and capsules of doxycycline hyclate were found to produce comparable serum levels of antibiotic activity when administered at recommended doses every 12 hours.