The concentration-effect relationships of carbachol were studied for three different responses of detrusor muscle of rabbit urinary bladder: the mechanical response, the increase of 45Ca uptake and the displacement of [3H]quinuclidinyl benzilate (QNB) from muscarinic receptors. The EC50 for carbachol in production of contractile responses of the detrusor muscle was observed to be 0.27 microM. The 45Ca uptake by detrusor muscle strips was studied by the lanthanum-residual wash out method. Carbachol increased 45Ca uptake with an EC50 of 12.7 microM. [3H]QNB binding was performed on homogenates of rabbit bladder detrusor muscle. By Scatchard analysis, the receptor density was found to be 47.2 fmol/mg of protein and the KD 0.074 nM. The contractile responses, the 45Ca uptake and the specific [3H]QNB binding were all blocked by atropine and so are taken to represent muscarinic receptor-mediated phenomena. Carbachol displacement of [3H]QNB yielded an EC50 of 135 microM and a Kl of 42 microM. It is concluded that the mechanical response requires a much lower concentration of carbachol than that needed for stimulating 45Ca uptake or for occupying [3H]QNB binding sites and that there appears to be approximately a 150-fold muscarinic receptor excess.