On page 313, Nagai et al. describe a way of visualizing phosphorylation activity of cAMP-dependent protein kinase (PKA) as it occurs in living cells. They placed a well-characterized substrate of PKA between blue and green mutants of GFP Phosphorylation of this peptide by PKA resulted in a conformational change that reduced fluorescence energy transfer (FRET) between the two GFPs. In COS-7 cells expressing the indicator, the reduced activity of PKA after treatment with kinase inhibitors could be monitored by FRET, suggesting that the fluorescent indicator might be useful in screens for kinase inhibitors.