The behavior of a massive star during its final catastrophic stages of evolution has been investigated theoretically, with particular emphasis upon the effect of electron-type neutrino interactions. The methods of numerical hydrodynamics, with coupled energy transfer in the diffusion approximation, were used. In this respect, this investigation differs from the work of Colgate and White (1964) in which a "neutrino deposition" approximation procedure was used. Gravitational collapse initiated by electron capture and by thermal disintegration of nuclei in the stellar center is examined, and the subsequent behavior does not depend sensitively upon which process causes the collapse.As the density and temperature of the collapsing stellar core increase, the material becomes opaque to electron-type neutrinos and energy is transferred by these neutrinos to regions of the star less tightly bound by gravity. Ejection of the outer layers of the star can result. This phenomenon has been identified with supernovae.Uncertainty concerning the equation of state of a hot, dense nucleon gas causes uncertainty in the temperature of the collapsing matter. This affects the rate of energy transfer by electron-type neutrinos and the rate of energy lost to the star by muon-type neutrinos.The effects of general relativity do not appear to become important in the core until after the ejection of the outer layers.