Atmospheric effects of precipitating solar wind protons in the Martian atmosphere are studied. The proton flux to the atmosphere is derived from a newly developed global quasineutral hybrid simulation which includes solar wind H+ ions and planetary O+ ions. The motion of the precipitating particles in the atmosphere is followed, and the effects of collisions to atmospheric neutrals are studied by a collision-to-collision Monte Carlo algorithm. Maximum atmospheric effects are estimated by using a fully absorbing boundary condition in the hybrid model where all solar wind protons are allowed to precipitate into the atmosphere without reflection. The developed mass-loaded hybrid code is found to reproduce many of the observed plasma and field features near Mars. When the vertical profiles of the energy deposition rates, CO2+ ionization rates, and Lyman alpha emission rates are calculated at different solar zenith angles, the maximum atmospheric effects on the dayside under average solar wind conditions are found to be typically a few percent of the effects of EUV radiation. On the nightside the proton precipitation is estimated to be intensive enough to be able to produce the measured ionospheric electron densities. The analysis illustrates that the atmospheric effects are strongly coupled with the global plasma interaction process between Mars and the solar wind. ¿ 2001 American Geophysical Union