Foliar urea application on barley plants increased leaf urease activity for 5 hours with a peak of 20-fold at 2 hours. To discern the mode of urease induction, urea with or without inhibitors and [35S]methionine were incubated with leaf sections for different lengths of time. Urease was extracted, partially purified, electrophoresed, and then quantified by fluorogram. Five urease (U) isozymes were separated by PAGE. Ua and Ub might be polymers or complexes that occurred only at the peak of induced activity. U1 and U2 appeared at 0.5 and 0.75 hour, respectively, after urea induction, peaked at 2 hours, and persisted only in treated leaves for several additional hours indicating that they are transient inducible forms. U3 was the constitutive form present in control and treated leaves. Induction with cordycepin or cycloheximide completely prevented urea stimulated activity and nullified the existence of isozymes Ua, Ub, U1, and U2. 35S-U1, which was labeled in the last hour of induction, appeared on fluorogram 1 hour after induction, peaked at 2 hours, and declined at 3 hours. Results indicated that de novo synthesis of urease is activated by the influx of urea.