Kinetic isotope effects for the reactions of Zr (4d(2)5s(2),F-3) with C2H4 and C2D4 and with C3H6 and C3D6 are measured in a fast now reactor at 300 K with He/N-2 buffer gas at 0.8 Torr. The H-2 and D-2 elimination products are detected using single photon ionization at 157 nm and time-of-night mass spectrometry. We find no significant isotope effect for reaction with either ethylene or propylene, These results clearly favor an indirect mechanism involving addition of the metal atom to the CC double bond and subsequent CH insertion and rule out a recently proposed mechanism involving direct CH bond insertion at low collision energy. Density functional theory in its B3LYP/LANL2DZ form finds no barrier to addition of ground state Zr to the double bond of ethylene and confirms the existence of a low-energy path from the metallacyclopropane complex to H-2 products. Theory also provides a realistic set of geometries and vibrational frequencies for use in a statistical rate model of the hot metallacyclopropane complex decay. However, the RRKM calculations indicate that a small barrier (0.5-2 kcal/mol) to the approach of Zr and ethylene is necessary to explain our kinetics data and the crossed beam angular distributions of Willis et al. In addition, the barrier to CH insertion must be lowered by 3-6 kcal/mol from the B3LYP/LANL2DZ value.