ZnO nanorods grown via vapor transport methods have been studied under high optical excitation. Via a focused ion beam (FIB), particular nanorods have been marked on the substrate for optical measurements on as-grown single rods. In the particular nanorods, stimulated emission occurs above a certain excitation threshold. A time-resolved measurement of the single nanorod emission is accomplished by the use of a streak-camera. We focus on the typical lasing dynamics, which is found in these measurements on several samples. We explain the complex dynamics after pulsed excitation, which is caused by the interplay of gain competition among several lasing modes, the bandgap renormalization, and the carrier-density-dependent refractive index.