Internal motions of two 14-nt RNA hairpins containing UUCG and GCAA tetraloops were studied by C-13 NMR relaxation and molecular dynamics. While both tetraloops exhibit an increased mobility on the fast time scale, with the GCAA loop we have detected a significant contribution of conformational dynamics on the millisecond to microsecond time scale. This is consistent with the observation that the GNRA family of tetraloops appears more flexible and tolerant of the conformational changes important for molecular recognition.