Maleimido-fluoroprobe (MFP) is a derivative of "fluoroprobe" (FP), the most sensitive known fluorescent polarity probe. MFP differs from FP in that it is completely nonfluorescent. The characteristic strong FP-type fluorescence is however switched on when the C=C bond of the maleimide group becomes saturated forming the succinimide derivative, SFP. This fluorogenic behavior is shown to be induced by copolymerization of MFP into growing polyacrylate chains. The increase in the intensity of the fluorescence, which is observable even in the early, low-viscosity stage of polymerization, provides therefore a measure of the degree of polymerization. The strong blue shift of the fluorescence of chain-incorporated SFP moieties fulfills the usual polarity probe function of monitoring changes in the microscopic dielectric environment of the polymer as vitrification takes place. This duality in the probe characteristics of MFP is illustrated by results on the radiation-induced bulk polymerization of methyl methacrylate.