We investigated effects of the geometry and size of the cerebrospinal fluid (CSF) on transmit and safety efficiencies in magnetic resonance imaging at 300 MHz. For the human model investigated here and a head position at the top of the RF coil, an increase of CSF space resulted in essential degradation of the safety excitation efficiency without an easily recognizable influence on the magnetic field distribution. For a head location in the coil center, CSF space variation did not significantly change the safety excitation and transmit efficiencies. We note that the assumed geometries in our study may not reflect in vivo conditions precisely and the results cannot be readily generalized.