The presence of antibiotic residues in pig tissues requires a search for new methods for their antemortem detection. To find an alternative for postmortem pig carcass analysis, an oral fluid was tested. To prove the suitability of oral fluid for the detection of antibiotics administered by injection, oxytetracycline was chosen. Research was conducted on two groups of animals: group 1, 100% treated; and group 2, 50% treated and 50% untreated. Oxytetracycline was assayed by a high-performance liquid chromatography- Tandem mass spectrometry method. The antibiotic was detectable 2 h post administration in group 1 and group 2 at the concentrations of 10653 ± 1421 μg/kg and 7457 ± 1145 μg/kg, respectively. At withdrawal period (21st day), oxytetracycline concentrations in oral fluid (30.8 ± 9.4 μg/kg in group 1 and 11.6 ± 5.6 μg/kg in group 2) were similar to those determined in muscle (34.5 ± 8.2 μg/kg). The concentrations of oxytetracycline in liver and kidney were 76.8 ± 22 μg/kg and 204 ± 49 μg/kg, respectively. The results of this study indicate that oral fluid analysis can be used for antemortem oxytetracycline detection in pigs, even if the half of animals in one pen are treated.