Caged ATP was photolysed in rat psoas fibres under various conditions to examine whether ADP plays a special role in the ‘Bremel–Weber type cooperation’, viz. the Ca2+-like action of the rigor cross-bridges. Various concentrations of ATP (0.25–1.6mm) were photoreleased in the presence of various concentrations of ADP (0–0.4mm) at ∼8 or 20°C. The Ca2+ concentration was set at around 0.2μm in order that the ATP-induced contraction was significant but short. Both lower [ATP] and higher [ADP] resulted in a slower detachment of the rigor cross-bridges and a larger contraction after the detachment. ADP did not seem to affect the relationship between the rate of detachment and the size of the Ca2+-sensitive component of the subsequent contraction. It is concluded that there is little evidence that the ADP-bound rigor cross-bridges are more potent than the nucleotide-free ones in the Bremel–Weber type cooperation that is seen in the ATP-induced transient contraction. The mechanism by which the ADP-specific, Ca2+-independent contraction occurs immediately after the release of ATP remains to be clarified.