Structural and dynamic studies of the lac Operator complexed with the headpiece of the lac Repressor are necessary to establish whether the two partners have pre-required structures or if folding is essential to induce the specific interaction. NOESY spectra of the 1 : 1 complex of half of the lac Operator 5′d(CGCTCACAATT)–5′d(AATTGTGAGCG) sequence (selectively labelled with 13C at C-1′), with the lac Repressor N-terminal headpiece 1–51 yielded information about the contact points between the nucleic acid and the protein. The operator was selectively labelled with 13C at the C-1′ position, allowing the measurement of 13C relaxation rates as a probe of dynamic behaviour. Measurement of the relaxation rates of R(Cz), R(Cx,y) and R(Hz → Cz) within the complex were carried out and a sequence dependence of R(Cx,y) was observed. The different possible relaxation processes were analysed and the data were interpreted in terms of chemical exchange between two conformations of the operator. It is concluded that the complexation of the N-terminal headpiece 1–51 with the lac Operator induced conformational changes of the nucleic acid. These data, in combination with reported 15N relaxation rates, suggest that the mobility of some regions of the nucleic acid and of the protein are essential for effective interaction between the two molecules. Copyright © 2000 John Wiley & Sons, Ltd.