1. Three benzoquinones (p-benzoquinone, 2,5-dimethyl-p-benzoquinone and tetramethyl-1,4-benzoquinone), differing for their electrophilicity, were tested on Ca2+ ATPase activity of cardiac sarcoplasmic reticulum membrane vesicles. 2. Only p-benzoquinone and 2,5-dimethyl-p-benzoquinone inhibited Ca2+ ATPase activity in a time-and concentration-dependent way, tetramethyl-1,4-benzoquinone being ineffective. 3. p-Benzoquinone and 2,5-dimethyl-p-benzoquinone decreased in a concentration-dependent way the total--SH groups of cardiac sarcoplasmic reticulum vesicles. 4. The results suggest that the most electrophilic benzoquinones interact directly with some critical--SH groups of Ca2+ pump inhibiting the enzymatic activity.