Time resolved fluorescence was used to study the dynamics on the nanosecond and subnanosecond time scale of the peptide hormone motilin. The peptide is composed of 22 amino acid residues and has one tyrosine residue in position 7, which was used as an intrinsic fluorescence probe. The measurements show that two rotational correlation times, decreasing with increasing temperature, are needed to account for the fluorescence polarization anisotropy decay data. Viscosity measurements combined with the fluorescence measurements show that the rotational correlation times vary approximately as viscosity with temperature. The shorter rotational correlation time (0.08 ns in an aqueous solution with 30% hexafluoropropanol, HFP at 20 degrees C) should be related to internal movement of the tyrosine side chain in the peptide while the longer rotational correlation time (2.2 ns in 30% HFP at 20 degrees C) describes the motion of the whole peptide. In addition, the interaction of motilin or the derivative motilin (Y7F) -23W (with tyrosine substituted by phenylalanine and with a tryptophan fluorophore added to the C-terminal) with negatively charged phospholipid vesicles (DOPG) was studied. The results show the development of a long anisotropy decay time which reflects partial immobilization of the peptide by interaction with the vesicles.