Since extracts from the plants Rhois aromatica and Solidaginis virgaurea are being used in the phytotherapy of bladder dysfunction including the overactive bladder syndrome, and since muscarinic receptors are the main pharmacological target in the treatment of bladder dysfunction, we have investigated whether these extracts can inhibit carbachol-induced, muscarinic receptor-mediated contraction of rat and human bladder. In vitro contraction experiments were performed with rat and human bladder strips. Radioligand binding and inositol phosphate accumulation studies were done with cells transfected with human M(2) or M(3) muscarinic receptors. Both extracts concentration-dependently (final concentrations 0.01-0.1%) inhibited carbachol-induced contraction of rat and human bladder with insurmountable antagonism. Radioligand binding experiments and inositol phosphate accumulation studies with cloned receptors demonstrated direct but non-competitive effects on muscarinic receptors. Reductions of KCl-induced bladder contraction demonstrated that inhibition by the higher extract concentrations also involved receptor-independent effects. We conclude that extracts from Rhois aromatica and Solidaginis virgaurea inhibit muscarinic receptor-mediated contraction of rat and human bladder. While this could contribute to the beneficial effects of these extracts in patients with bladder dysfunction, such therapeutic effects remain to be demonstrated in controlled clinical studies.