Simple SummaryDoxycycline, an approved aquacultural antibiotic, is extensively used in the treatment of bacterial diseases in fish. Since fish are poikilothermic organisms, their body temperature and metabolic rate are primarily influenced by the temperature of the water. Therefore, temperature may be affected by pharmacokinetic behavior and withdrawal times of drugs. The current study was undertaken to look at the differences in pharmacokinetics, tissue residues, and withdrawal times of doxycycline following oral administration in rainbow trout reared at 10 and 17 °C. The increment of water temperature from 10 to 17 °C decreased the elimination half-life, the body clearance, and the distribution volume of doxycycline and increased plasma concentrations. The withdrawal times for plasma and tissues decreased with the temperature increase. The results contributed to the determination of an optimal dosing regimen and the safe consumption of edible tissues in rainbow trout that were administered doxycycline and reared at different temperatures.AbstractThe purpose of this study was to compare the pharmacokinetics, tissue residues, and withdrawal times of doxycycline after oral administration in rainbow trout reared at 10 and 17 °C. Fish received a 20 mg/kg oral dose of doxycycline after a single or 5-day administration. Six rainbow trout were used at each sampling time point for plasma and tissue samples, including liver, kidney, and muscle and skin. The doxycycline concentration in the samples was determined using high-performance liquid chromatography with ultraviolet detector. The pharmacokinetic data were evaluated by non-compartmental kinetic analysis. The WT 1.4 software program was used to estimate the withdrawal times. The increase of temperature from 10 to 17 °C shortened the elimination half-life from 41.72 to 28.87 h, increased the area under the concentration–time curve from 173.23 to 240.96 h * μg/mL, and increased the peak plasma concentration from 3.48 to 5.50 μg/mL. At 10 and 17 °C, the doxycycline concentration was obtained in liver > kidney > plasma > muscle and skin. According to the MRL values stated for muscle and skin in Europe and China (100 μg/kg) and in Japan (50 μg/kg), the withdrawal times of doxycycline at 10 and 17 °C were 35 and 31 days, respectively, for Europe and China and 43 and 35 days, respectively, for Japan. Since temperature significantly affected pharmacokinetic behavior and withdrawal times of doxycycline in rainbow trout, temperature-dependent dosing regimens and withdrawal times of doxycycline might be necessary.