Liver and muscle tissue residues of doxycycline in turkeys were determined following administration of 25 mg doxycycline x HCl/kg BW in the drinking water under field conditions. Quantitation was performed using a validated HPLC method with fluorescence detection. The method was able to separate doxycycline and its 4-epimer, 4-epidoxycycline. This epimer was found in both liver and muscle tissue. The detection limits of the method were estimated at 1.2 ng/g and 1.0 ng/g of doxycycline in liver and muscle tissue, respectively, using a signal-to-noise ratio of 3:1. The recovery of doxycycline was determined from spiked tissues and was 63+/-3.8% and 66+/-3.1% for liver and muscle, respectively (n = 6). Within-day and between-day imprecision, expressed as the R.S.D. was below 7.4%. Linear calibration curves (r>0.997) were obtained in spiked liver between 0 and 1500 ng/g and in spiked muscle between 0 and 500 ng/g. A good stability of doxycycline was observed in liver samples after storage for 22 days at -20 degrees C. The correlation between the residues in the liver and the muscle was expressed as the correlation coefficient r and was 0.9884. The depletion kinetics of doxycycline fitted a one-compartment model. The elimination half-life (T1/2) of doxycycline was 77.7 h and 78.0 h in muscle and liver, respectively. Furthermore, the residue depletion kinetics were used to establish a withdrawal period in conformity with official guidelines. The withdrawal times necessary to reach concentrations below maximum residue limits (MRLs), as imposed by the EU, were 12 days and 17 days for liver and muscle tissue, respectively.