Slow‐twitch mechanically skinned fibres from rat soleus muscle were bathed in solutions mimicking the myoplasmic environment but containing different [ADP] (0.1 μm to 1.0 mm). The effect of ADP on sarcoplasmic reticulum (SR) Ca2+‐content was determined from the magnitude of caffeine‐induced force responses, while temporal changes in SR Ca2+‐content allowed determination of the effective rates of the SR Ca2+‐pump and of the SR Ca2+‐leak. The SR Ca2+‐pump rate, estimated at pCa (−log10[Ca2+]) 7.8, was reduced by 20% as the [ADP] was increased from 0.1 to 40 μm, with no further alteration when the [ADP] was increased to 1.0 mm. The SR Ca2+‐leak rate constant was not altered by increasing [ADP] from 0.1 to 40 μm, but was increased by 26% when the [ADP] was elevated to 1.0 mm. This ADP‐induced SR Ca2+‐leak was insensitive to ruthenium red but was abolished by 2,5‐di(tert‐butyl)‐1,4‐hydroquinone (TBQ), indicating that the leak pathway is via the SR Ca2+‐pump and not the SR Ca2+‐release channel. The decrease in SR Ca2+‐pump rate and SR Ca2+‐leak rate when [ADP] was increased led to a 40% decrease in SR Ca2+‐loading capacity. Elevation of [ADP] had only minor direct effects on the contractile apparatus of slow‐twitch fibres. These results suggest that ADP has only limited depressing effects on the contractility of slow‐twitch muscle fibres. This is in contrast to the marked effects of ADP on force responses in fast‐twitch muscle fibres and may contribute to the fatigue‐resistant nature of slow‐twitch muscle fibres.