Forces acting between tunnel tip and sample during scanning tunneling microscopy were explored experimentally. Force gradients were detected as resonance shifts of cantilever beams used as sample stage. Beam deflections caused by force variations were also recorded. We present data for tungsten tips and polycrystalline silver surfaces prepared under UHV conditions. Large positive force gradients were found for the range of tunnel distances investigated. Force maps show pronounced features correlating directly with topographic images. © 1986 The American Physical Society.