1. ADP inhibits the maximum shortening velocity (V0) in skeletal muscle. [ADP] may increase considerably during contractions and reduce V0 in the absence of energy buffering by phosphocreatine (PCr). We have tested this hypothesis by comparing V0 in long and short tetani produced in situations where PCr buffering is absent. 2. Single, intact muscle fibres were dissected from toe muscles of Xenopus and stimulated by current pulses at 20 degrees C. The test sequence consisted of a 400 ms tetanus, followed after 3 s by a 1400 ms tetanus and after an additional 4 s by a 400 ms tetanus. V0 was measured with slack tests at 200 and 1200 ms, respectively. 3. The PCr system was inactivated in three ways: (i) fatiguing fibres with repeated short tetani; (ii) inhibition of the creatine kinase (CK) reaction with dinitrofluorobenzene; and (iii) inhibition of energy metabolism with iodoacetic acid and cyanide. 4. Under control conditions V0 was similar in all three test tetani. With inactive PCr buffering V0 was about 30% lower in the long tetanus. This slowing recovered fully in the second short tetanus in fatigue and with CK inhibition. 5. Calculations suggest that [ADP] can reach very high levels (about 3 mM) during prolonged contractions in the absence of PCr buffering.