The present study was undertaken to compare the effects of the thiol reagents L-cysteine and (diazene dicarboxylic acid bis 5N,N-dimethylamide) diamide on contractile activity of neonatal and adult rat bladders. In vitro whole-bladder preparations from Wistar rats were used to study the modulation of spontaneous bladder contractions by thiol reagents. After blocking cholinergic and adrenergic transmission with atropine and guanethidine, L-cysteine facilitated spontaneous bladder contractions in neonatal rat bladders. The effect of L-cysteine was suppressed by diamide. Diamide alone did not change basal activity of the neonatal rat bladder. The facilitatory effects of L-cysteine were reduced by the L-type Ca2+ channel-blocking agent nifedipine and the calcium-activated K+ channel opener NS1619 [1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl) phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one]. ATP or suramin, a purinergic receptor antagonist, significantly inhibited the effect of L-cysteine in neonatal bladders, whereas the nitric-oxide synthase inhibitor N-omega-nitro-L-arginine was ineffective. L-cysteine did not elicit any detectable effects in the adult rat bladder; whereas diamide caused a large-amplitude sustained tonic contraction. The contraction induced by diamide in adult bladder did not occur when the preparation was pretreated with L-cysteine. Also, L-Cysteine administered during the diamide-evoked contraction completely inhibited the contraction to diamide. In conclusion, our results suggest that L-cysteine has markedly different effects in isolated whole-bladder preparations from neonatal and adult rats. Thus thiol-sensitive mechanisms may modulate contractility by regulation of Ca2+ and K+ channels and/or purinergic transmission in the neonatal bladder. The effects of L-cysteine and diamide were reversed in adult bladders, indicating that the regulation of bladder contractility by thiols is markedly altered during postnatal development.