Bladder instability is a prevalent condition, especially in the elderly. However, despite its recognition as the greatest cause of urinary incontinence the aetiology is poorly understood. Bladder smooth muscle (detrusor) obtained from some patients with bladder instability exhibit atropine-resistant nerve-mediated contractions, in contrast to detrusor from stable bladders. These contractions are blocked by purinoceptor antagonists suggesting the presence of a second neurotransmitter, ATP. This study was undertaken to characterise further purinergic neuromuscular transmission in human and guinea pig detrusor and its relationship to the development of detrusor instability in patients. The contractile sensitivity of muscle from stable and unstable bladders to P2- purinoceptor agonists was measured using ATP and the non-hydrolysable analogues α,β-methylene-ATP and β,γ-methylene-L-ATP. ATP was significantly more potent in detrusor from unstable or obstructed bladders than from stable bladders; α,β-methylene- ATP was more potent than ATP in all tissues, β,γ-methylene-L-ATP was also more potent in guinea-pig tissue. Ecto-ATPase activity, measured using the bioluminescent protein luciferase, was lower in detrusor samples obtained from unstable compared to stable bladders. The involvement of inhibitory P1-purinoceptors in modulating nerve-mediated contractions and agonist-induced contractures was also investigated. Detrusor preparations from unstable and obstructed bladders were less sensitive to the synthetic adenosine analogue, N-ethylenecycloadenosine, than those from stable bladders. The actions of adenosine were similar in all detrusor samples tested. In experiments using several P1-receptor subtype-selective agonists and the P1-receptor antagonist, 1,3- dipropyl-8-cyclopentylxanthine, in guinea pig detrusor the presence of a pre-synaptic A1 and a postjunctional A2B receptor was identified. In this study the role of purinergic agonists in regulating the contractility of detrusor muscle has been characterised. Differences were found in detrusor samples from patients with stable and unstable bladders. The implications of these findings in understanding neuromuscular transmission in the normal and unstable human bladder are discussed.