The dephosphorylation of adenine nucleotides and their analogues by ectonucleotidases on the guinea-pig urinary bladder was studied using HPLC. The rate of dephosphorylation of each analogue was compared with its pharmacological potency at causing contraction. ATP, ADP and AMP were rapidly dephosphorylated, and substitution on the purine ring did not affect the rate of breakdown. The ectonucleotidases showed stereoselectivity towards the ribose moiety and towards the polyphosphate chain. In general, methylene isosteres of the nucleotides, and analogues in which one of the oxygen atoms on the terminal phosphate had been replaced, were resistant to degradation. None of the analogues that were readily dephosphorylated are more potent than ATP, and most but not all of the analogues resistant to degradation are more potent than ATP, suggesting that while resistance to degradation does not in itself confer high potency, susceptibility to degradation does limit the potency of ATP and its degradable analogues.