Solid-phase microextraction probes based on poly(ethylene glycol)/C18-bonded silica were used for in vivo monitoring of drugs from circulating blood of beagles, over a period of 8 h. After sampling, the extracted drugs were subsequently quantified by liquid chromatography coupled with tandem mass spectrometry. External calibrations in whole blood and phosphate-buffered saline were used to correlate the amount of analytes extracted in regard to the total and free concentrations in blood respectively. The probe provided sufficient sensitivity for the drugs in the blood matrix, while the need for drawing blood was eliminated. The limit of detections of the method from whole blood were 1.7, 1.4 and 2.8 ng mL⁻¹ for the analysis of diazepam, nordiazepam and oxazepam respectively, and the linear range was from 4 ng mL⁻¹ to 2 μg mL⁻¹. The method was applied for the monitoring of pharmacokinetic profiles of intravenous administration of diazepam and its two main metabolites in dogs, and the results were compared with profiles determined by conventional methods. This approach offered increased sensitivity and accuracy, short extraction time, and convenient calibration for in vivo sampling for dynamic monitoring.