The second-order nonlinear optical properties of green fluorescent proteins (GFPs), such as the photoswitchable Dronpa and enhanced GFP (EGFP), have been studied at both the theoretical and experimental levels. In the case of Dronpa, both approaches are consistent in showing the rather counterintuitive result of a larger second-order nonlinear polarizability (or first hyperpolarizability, β) for the protonated state, which has a higher transition energy, than for the deprotonated, fluorescent state with its absorption at lower energy. Moreover, the value of β for the protonated form of Dronpa is among the highest reported for proteins. In addition to the pH dependence, we have found a wavelength dependence in the β values. These properties are essential for the practical use of Dronpa or other GFP-like fluorescent proteins as second-order nonlinear fluorophores for symmetry-sensitive nonlinear microscopy imaging and as nonlinear optical sensors for electrophysiological processes. An accurate value of the first hyperpolarizability is also essential for any qualitative analysis of the nonlinear images.