Glycinexylidide (GX) is a metabolite of lidocaine that is frequently present in mug/ml concentrations in the plasma of patients treated with lidocaine infusions for 24 hr or more. Plasma levels of GX have 26% the antiarrhythmic activity of lidocaine in an animal model, and GX adversely affects the mental performance of normal subjects at plasma concentrations comparable to those found in patients. The total volume of GX distribution in man is similar to that of lidocaine but the plasma clearance is less, so that the 10-hr elimination phase half-life of GX is much longer than the 1 1/2 hr half-life reported in normal subjects for lidocaine. About half of an administered dose of GX is excreted unchanged in urine, roughly 15% appears in urine as conjugates of xylidine and p-OH xylidine, and the fate of the rest is unknown.