Plant growth regulators (PGRs) are extensively used for growth manipulation of turfgrasses. PGR responses can vary with season, temperature, and other environmental factors. This research project examined the metabolism of two widely used PGRs, trinexapac-ethyl and paclobutrazol, in two different turfgrass species at two times during the growing season. Trinexapac-ethyl and paclobutrazol were applied to pure stands of annual bluegrass or creeping bentgrass maintained at 1.3 cm height of cut in May and July of 2005. Clippings were collected following application and residue levels of the PGRs determined by liquid chromatography with UV detection. Trinexapac-ethyl is rapidly metabolized by both species at similar rates. Paclobutrazol metabolism differed by species with creeping bentgrass metabolizing paclobutrazol more rapidly than annual bluegrass. The differences in metabolism between the two species is responsible for the differential growth suppression that allows paclobutrazol to be used like a herbicide to reduce the population of annual bluegrass.