Bovine plasma albumin (BPA) has approximately one SH group (Cys‐34) which catalyzes the intramolecular SH, S‐S exchange reaction in the alkaline region at low ionic strength, resulting in the formation of the aged form. So, the N‐B transition at ionic strength above 0.20 and below 0.10 was studied using BPA and iodoacetamide‐blocked BPA (IA‐BPA), respectively. (1) pH profiles of [θ]262and[θ]268of BPA in 0.20mKCl showed the characteristic changes in the pH region 7.0‐9.0, corresponding to the N‐B transition. On going from pH 7.0 to 9.0 in 0.10 M KCl or NaCl, IA‐BPA did not show significant changes in rotational relaxation times of tryptophyl fluorophors, CD‐resolved secondary structures, spin‐echo1H‐n.m.r. spectra and cross‐relaxation times (T1S) between irradiated and observed protein protons, which might reflect the rigidity of the domains and/or subdomains. On the other hand, rotational relaxation times of l‐anilino‐8‐naphthalenesulfonate‐IA‐BPA complex (IA‐BPA‐ANS0 9, molar ratio of ANS to IA‐BPA = 0.9/1) showed significant decreases from 131 to 114ns on going from the N‐ to the B‐forms in 0.10M KCl. The above results and reported experimental evidence might indicate that on going from the N‐ to the B‐forms in 0.10mKC1 or NaCl, the mutual movement of subdomains, connected with a flexible hinge region (Brown&Shockley (1982)) might increase without loss in the helicity and the rigidity of subdomains. (2) The N‐B transition of IA‐BPA in the absence of salt was quite different from those in 0.10 M KCl or NaCl. Decreases in the helicity and the intramolecular rigidity, as monitored by TIS‐measurem