We investigate the effects of solar energetic particle (SEP) events on the Martian ionosphere using observations from the Mars Global Surveyor (MGS) Electron Reflectometer (ER) and Radio Science (RS) experiments. Although MGS/ER is not designed to measure solar storm particles, it detects SEPs as increased instrument background. Using this proxy for SEP fluxes near Mars, we compare electron density profiles obtained from the RS experiment during periods of high and low SEP activity. Six case studies show no clear evidence for an increase in the ionospheric electron density between 200 and 100 km altitudes. However, 4 of the 6 events show a small increase in electron density below 100 km altitude during SEP events, suggesting that high-energy (10-20 keV) electrons may cause ionization in the lower ionosphere. We also observe an similar to 25% decrease in the ionospheric electron density between similar to 100 and similar to 120 km altitude for the two strongest events, suggesting that SEPs trigger a process that increases electron loss in this altitude range of the lower ionosphere. However, we cannot be confident from only two events that this effect is caused directly or indirectly by increased SEP fluxes. A statistical study confirms the case study results, but not over all solar zenith angles. Additionally, we observe depletions in the topside ionospheric electron density at some solar zenith angles, which can be explained by compression of the ionosphere by the passing CME. Citation: Ulusen, D., D. A. Brain, J. G. Luhmann, and D. L. Mitchell (2012), Investigation of Mars' ionospheric response to solar energetic particle events, J. Geophys. Res., 117, A12306, doi: 10.1029/2012JA017671.