Diffusion effects influence magnetic resonance imaging (MRI) in quite diverse ways. In this contribution, a short survey is provided on how diffusion directly and indirectly affects the contrasts observable in MR images. Special attention is given to the interaction of diffusion and longitudinal magnetization in thin excited slices. Results of experimental studies on the NMR signal amplitude in periodically excited slices with thicknesses in the range of a few mu m are presented. Signal enhancements by more than a factor of 100 were observed. The effects studied here may have a great impact on contrast and signal/noise ratio observed in MRI at ultrahigh resolution or in certain implementations of NMR force microscopy.