The replacement of one of the carbonyl groups in a 1,2,3-triketone-2-naphthylhydrazone with a pyridine ring yields an original molecular switch that can be switched fully, effectively, and reversibly between the E and Z configurations. This hydrazone-based, pH-controlled, molecular switch is the first example of a chemically controlled configurational rotary switch. The bistable switch exists primarily (97%) as the E configuration in solution and can be converted quantitatively to the Z-H+ configuration upon treatment with trifluoroacetic acid. When Z-H+ is passed over a plug of K2CO3, the "metastable" Z configuration is observed using H-1 NMR spectroscopy, which thermally equilibrates to give back the E configuration. The rate of this process is dependent on the polarity of the solvent, indicating that the E/Z isomerization takes place via a rotation around the hydrazone C=N bond.