With quick-freeze deep-etch electron microscopy coupled with mica-flake technique, I showed previously that myosin subfragment-1 (S1) attached to F-actin in the presence of ATP is short and rounded, in contrast to its elongated and tilted appearance under rigor condition [J. Biochem. 106, 751-770 (1989)]. I further indicated that each head of heavy meromyosin (HMM) changes its configuration in a likely manner as above by the addition of various nucleotides, i.e. heads were pear-shaped in the absence of nucleotide, in a ball-on-a-stick appearance when complexed with ADP and strongly kinked to the particular direction in the presence of ATP or ADP.Vi [J. Muscle Res. Cell Motility 12, 313 (1991)]. Such morphological data not only corroborates the independent biophysical evidences suggesting gross conformational changes of myosin head upon binding ATP or ADP.Vi, but also provide strong evidence for the distinct polarity in the structure of each myosin head. Negatively stained image of chemically cross-linked acto-S1 also included cross-bridges sharply kinked to the same direction, confirming the above observation. Attempts were made to examine if such conformational change of myosin cross-bridge occurs during actomyosin superprecipitation. Samples were quick-frozen during rapid turbidity-increasing phase where actin filaments actively slide past myosin heads. The resultant image included actin-attached myosin heads all in a kinked configuration with the same polarity as observed for HMM. Several heads associated with a single actin filament were bent to the same direction suggesting that myosin heads might be in a kinked configuration with distinct polarity during contraction.