A rapid flow injection (FI) spectrophotometric procedure for the determination of tetracycline(TC), chlortetracycline(CTC) or oxytetracycline(OTC) has been developed. The method is based on the injection of 150 μL standard or sample solution into a reagent stream of iron (III) (500 μg/mL in 5.0 × 10-3 mol/L HNO3) with the optimum flow rate of 4.0 mL/min. The brown iron (III)- tetracycline complex is monitored at 423 nm. The flow injection system and the experimental conditions were optimized by means of univariate method. Under the optimum conditions, calibration graphs were obtained for 1.0 - 100.0 μg/mL and the detection
limits (s/n = 3) were 0.1 μg/mL of each drug. The correlation coefficients were 0.995, 0.994 and 0.990 for tetracycline, chlortetracycline and oxytetracycline, respectively. The method was successfully applied to the determination of these drugs in pharmaceutical formulations with a sample throughput of 70/h.