The technique of Perturbed Angular Correlations of 7-rays has been used to study the rotational correlation times in aqueous solution of the peptides: oxytocin, glycyltryptophan, cholecystokinin and the glycopeptide ristocetin. These peptides were labelled with excited 1limed through the covalent coupling of the metal chelator diethylenetriaminepentaacetic acid (DT-PA) to the primary amines-of the peptides. The experimental correlation times are in good accordance with calculations based on the molecular weight. This indicates that the I IxmCd-DTPA is rigidly bound to the molecules. In the case of ristocetin, the correlation time was measured at 2 °C, 25 °C and 38 °C. These experiments show the expected linear dependence on the viscosity divided by temperature. The feasibility of determining rotational correlation times for peptides without lysines and with correlation times in the ns region is thus demonstrated. Also, the correlation time of ~ 1 lmCd-DTPA coupled to the lysines of bovine serum albumin was determined. The measured correlation time is about 5 times less than the calculated correlation time. This effect is assigned to local motion. In spite of this, experiments show that ~llmCd-DTPA-bovine-serum-albumin is significantly immobilised by aggregation with immunoglobulins. The nuclear quadrupole interactions, necessary for determining the correlations times, were determined for 11 lmCd_DTPA_ristocetin and 11 lmCd_DTPA_bovineserum-albumin by adding sucrose to a concentration of 63% and cooling to 2 °C. This showed a small but significant difference between the two molecules. We interpret this as due to different conformations, possibly different coordination numbers.