A solid-state NMR exchange technique for detecting slow segmental dynamics in proteins is introduced. The technique exploits motion-induced incomplete refocusing of amide N-15 chemical shift anisotropy under magic angle spinning. Slow motions on the millisecond timescales are detected as reduced NMR signals. Detection of C-13 magnetization transferred from N-15 allows the identification of mobile residues with high resolution. This exchange technique is demonstrated on two proteins with opposite motional properties. Combined with extensive C-13 and N-15 labeling, this high-resolution exchange NMR technique allowsfor the first time the efficient determination of slow dynamics at multiple residues of proteins.