The medial septum (MS) is crucial for the generation of theta rhythms, which support hippocampal memory formation and spatial navigation. To determine whether the MS acts as a ‘pacemaker’ or coordinates hippocampal activity patterns through reciprocal connections the authors cooled the MS in mice. Theta oscillation frequency and power were reduced, and errors in a spatial navigation task were increased, but there was no effect on spatial coding by individual place cells or the spatial map. This suggests that theta phase coordination is achieved by reciprocal MS–hippocampus connections.