Nitrate-selective microelectrodes based on a number of nitrate sensors were compared. The electrode properties tested included log-linear slope of the calibration curves, detection limit, and ion selectivity. The nitrate sensor mixture described in an earlier paper performed favourably when compared with other nitrate-selective mixtures or with commercially-available macroelectrodes. This earlier mixture consisted of 6% methyltridodecylammonium nitrate, 65% n-phenyloctyl ether, 23% poly(vinylchloride), 5% nitrocellulose, and 1% methyltriphenyl phosphonium bromide. These electrodes, even when stored backfilled remained nitrate-selective for several days although there was eventually some deterioration in performance. The electrodes were used in vitro for assaying nitrate in barley root extracts and a linear relationship was found between results from ion chromatography and microelectrode measurements. Intracellular measurements made in vivo in epidermal cells of excised barley roots identified two populations of measurements believed to be the cytoplasm and the vacuole. Significant decreases in compartmental nitrate activities were measured 2 to 6 h after excision. These results indicate that the nitrate pool in both the cytoplasm and vacuole of root epidermal cells is sensitive to root excision and question the physiological significance of measurements made on excised roots.