Objective. To develop enzyme activatable Förster resonance energy transfer (FRET) substrate probes to detect MMP-12 and MMP-13 activities in vivo in mouse models of inflammatory arthritis Methods. Peptidic FRET probes activated by MMP-12 and MMP-13 were reverse designed from inhibitors selected from a peptide phosphinic inhibitor library. Selectivity of the probes was demonstrated in vitro using MMP-1, MMP-2, MMP-3, MMP-12, and MMP-13. In vivo activation of the probe was tested in the zymosan-induced mouse model of inflammation and probe specificity was evaluated by the metalloprotease inhibitor GM6001 and specific synthetic inhibitors of MMP-12 and MMP-13. The probes were used to follow these enzyme activities in the collagen-induced arthritis (CIA) model in vivo. Results. The MMP-12- and MMP-13-activity probes (MMP12ap and MMP13ap, respectively) discriminated between the two enzymatic activities. The in vivo activation of these probes was inhibited by GM6001 and by their respective specific inhibitors. In the CIA model, MMP12ap activation peaked 5 days after disease onset and showed strong correlation with disease severity during this time (r = 0.85; p