A flow-gated, on-line interface between a microdialysis sampling probe and capillary zone electrophoresis with UV absorption detection was characterized and applied. Electrophoresis conditions were chosen so that ascorbic acid migrated in 42-100 s with 65 000-120000 theoretical plates. These results were obtained using a 25 pm inner diameter capillary with an inlet to detector length of 15 cm and electric field strength of 400-600 V cm-'. Theoretical plates, migration times, and peak areas for ascorbic acid injected on-line from the interface were the same as those obtained for off-line injections. The system allowed step changes in ascorbic acid concentration to be followed with 85 s time resolution when the dialysis flow-rate was 79 nl min-'. The time resolution was improved to 65 s when the dialysis flow-rate was 155 nl min-'. The relative standard deviation for peak heights was 3.8% and a linear response over the physiologically relevant range for ascorbic acid was observed. At 79 nl min-' the relative recovery of the probe was 98%. The high relative recoveries improved detection limits, simplified quantification, and resulted in decreased disturbance to the system being studied when compared to the more conventional dialysis flow-rates of 0.5-l ~1 min-'. As a demonstration of the system, ascorbic acid in the caudate nucleus of rat brain was detected and monitored in response to systemic amphetamine injections and anaesthetic overdoses. This system is the first to demonstrate high relative recoveries and high time resolution simultaneously with microdialysis sampling.