Thermotropic structural transitions in rabbit skeletal muscle heavy meromyosin and subfragment-1 (S-1) have been quantitatively investigated by using nucleotide-induced UV difference spectroscopy. The magnitude of the adenylyl 5'-imidophosphate (AMP-PNP)-induced difference spectrum is temperature-dependent for both S-1 and heavy meromyosin (HMM). The transition observed here appears to be the same transition observed by 31P NMR of bound AMP-PNP (Shriver, J., and Sykes, B. D. (1981) Biochemistry 20, 2004-2012). The ADP-induced spectrum is temperature-independent, which differs from the 31P NMR data, indicating that the chromophore contributing to the difference spectrum resides in a domain distinct from the active site, at least when ADP is bound. Although the magnitudes of the AMP-PNP-induced spectra are equal in magnitude for S-1 and HMM on a globular head basis, the temperature dependence of the AMP-PNP induced difference spectrum for S-1 differs significantly from that of HMM. The van't Hoff enthalpy for the apparent two-state transition in S-1 is half that observed with HMM: 19 (+/- 7.5) kcal/mol for S-1 and 35 (+/- 5) kcal/mol for HMM. This indicates an additional cooperative interaction in HMM which is not present in S-1. Modification of SH1 results in the loss of the temperature dependence of the AMP-PNP-induced difference spectrum, and the resulting difference spectra appear identical to those induced by ADP.