The hippocampus is critically involved in encoding masses of past experiences as distinct memories. It has long been hypothesized that the dentate gyrus minimizes overlaps among similar input patterns and imposes new patterns to learn onto CA3 network via strong mossy fiber synapses during memory encoding. Such ‘detonator’ hypothesis was directly tested by optogenetically stimulating mossy fibers in freely-behaving mice. We found that optogenetic stimulation of mossy fibers can drive CA3 neuronal firing in mice navigating along a circular track, but their effects are overall inhibitory and transient. Spatially restricted mossy fiber stimulation was more likely to suppress than enhance CA3 neuronal activity. Also, changes in spatial firing induced by optogenetic stimulation reverted immediately upon stimulation termination, leaving CA3 place fields unaltered. Our results argue against the view that mossy fibers convey teaching signals, and show the robustness of established CA3 spatial representations.