The pH dependence in the range 3--7 of the optical absorption and electron paramagnetic resonance of the nitric oxide adduct of ferrous Aplysia myoglobin is reported. Optical spectra in the Soret region show a transition between two conformers with an apparent pK in the range 3.5--5 depending on the presence of carboxylic anions as third component. In the same pH range, the EPR spectrum undergoes a change from a 9-line to a 3-line hyperfine pattern in the g. region, similar to that reported for synthetic heme derivatives and for other hemoproteins. The structural interpretation of the pH-induced transition experienced by Aplysia myoglobin nitric oxide is that of a proton-linked cleavage of the proximal bond as suggested by several lines of evidence. Temperature-jump measurements allowed an estimation of the relaxation time for the process, which is of the order of 0.3 ms at 25 degrees C.