Rotational correlation times have been determined from fluorescence anisotropy decays of the tyrosyl residue in the opioid pentapeptides DPDPE (Tyr-D-Pen-Gly-Phe-D-Pen), DPDPE(SH)(2), and [Leu(5)]-enkephalin, revealing internal peptide motions. Fluorescence decays were measured by time-correlated single-photon counting. For all three peptides, the fluorescence emission is characterized by three-exponential intensity decays with amplitudes that are consistent with ground-state populations of rotamers of the tyrosyl side chain. Rotational correlation times in water determined from single-exponential fits are 80-130 ps, in good agreement with molecular dynamics simulations [Wang, Y.; Kuczera, K. J. Phys. Chem. 1996, 100, 2555-2563]. Internal peptide motions were studied by measurement of the rotational correlation times in solutions of 50% propylene glycol in water over the temperature range from 258 to 313 K. Two distinct temperature regions were observed. In the low-temperature regime the thermal viscosity coefficient for each peptide is approximately 0.07 K-1, the same as for free tyrosine. Hence, in this temperature regime the rotational friction is imposed by the solvent alone, consistent with rigid-body rotational motion. At higher temperatures an additional source of reorientational motion is revealed by an apparent change in the thermal viscosity coefficient. The viscosity coefficient in the high-temperature regime is characteristic of the peptide and not just the solvent, indicating the influence of internal dynamics. Double-exponential fits yielded further evidence of internal tyrosyl reorientational motions, which make increasingly large relative contributions at higher temperatures.