Abstract： Objective To ascertain the existence of recovery time (RT) lag during microdialysis (MD) in vitro.Methods A specialized MD system for in vitro detection of free calcium ion was constructed.Perfusion was conducted by using normal saline (NS) at a rate of 2.0 μl/min.The dialysate was diluted synchronously and collected in a vial at 10-minute interv als for 8 times.The free calcium concentration ([Ca2+]) in the sample was measured by atomic absorption spectrophotometer for computation of the retrieval rate of MD system.Signal-switching between standard calcium solution and NS was employed to mimic changes in [Ca2+] in the target solution.This was followed by recording of [Ca2+] and RT regarding detection of [Ca2+] via MD system.Results A recovery rate of 16％ for MD system was revealed.The [Ca2+]increased at 10 min[(57± 17)μmol/L],peaked during steady-state at 20 min (200 μmol/L) and was wellmaintained till 50 min[(159±26)μmol/L)] followed by a subsequent reduction.An overall difference(all P＜0.05)in [Ca2+] between peak time points [20,30 and 40 min,for 20 min:(197±29) μmol/L,30 min:(194±21) μmol/L,40 min:(192± 11)μmol/L] and baseline time points[0,60,70 and 80 min,for 0 minute:(3±4)μmol/L,60 min:(7±8)μmol/L,70 min:(7±7)μmol/L,80 min:(5±9)μmol/L]was noted.The increase and decrease in [Ca2+] in the dialysate showed a noticeable time lag as compared with the target solution.An RT of 20 minutes corresponded to measurement of [Ca2+] by using the MD system.Conclusion The time lag for the MD system to detect [Ca2+] is measurable at l0-minute intervals.This should not warrant precise estimation prior to the measurement.The signal-switching technique proves applicable for RT calibration.