The MM2 and the new MM3 force fields have been extended to include hydrocarbons containing cyclopropane rings. In addition to simple cyclopropyl systems, many highly strained structures have been examined. The calculated structures are in a reasonable agreement with experiment, giving a 0.004-Å (MM2, standard deviation) and 0.003-Å (MM3) difference from the average C-C bond length obtained from gas electron diffraction analysis, for a set of 17 compounds. The MM3-calculated moments of inertia are within 0.7% (SD) of the microwave results. Enthalpies of formation have also been fit by using all available experimental data (19 compounds), ranging from −8 to +136 kcal/mol. The standard deviations between the calculated and the experimental values are 0.44 and 0.28 kcal/mol, for MM2 and MM3, respectively.