1. We investigated muscarinic receptors in the detrusor and mucosa of the human bladder body. Radioligand-binding studies with [(3)H]QNB were conducted using specimens collected from patients (36-77 years) with normal bladder function, undergoing surgery. For RT-PCR, biopsies of normal bladder were obtained from patients (30-88 years) undergoing check cystoscopy. 2. Binding of [(3)H]QNB in detrusor (n=20) was of high affinity (K(D) 77.1 (55.2-99.0) pM) and capacity (B(max) 181+/-7 fmol mg protein(-1)). Similar values were obtained in mucosa (n=6) (K(D) 100.5 (41.2-159.9) pM; B(max) 145+/-9 fmol mg protein(-1)). 3. Competition-binding experiments in detrusor membranes with muscarinic receptor antagonists including trospium, darifenacin, 4-DAMP, methoctramine, AQ-RA 741, AF-DX 116 and pirenzepine indicated a receptor population of 71% M(2), 22% M(3) and 7% M(1). In the mucosa, 75% of sites were M(2) receptors, with 25% M(3)/M(5). 4. Using RT-PCR, expression of M(1), M(2), M(3) and M(5) mRNA was demonstrated in both detrusor and mucosa. 5. The presence of a high density of mainly M(2) muscarinic receptors in the mucosa appears to be a novel finding and raises the question of their physiological significance and the source of their endogenous ligand. 6. There was a negative correlation of receptor number (B(max)) with age in detrusor muscle from male patients (P=0.02). Quantitative competitive RT-PCR demonstrated a selective age-related decrease in mRNA for muscarinic M(3) but not M(2) receptors, in both male (P<0.0001) and female (P=0.019) detrusor. These findings correspond with reports of decreased detrusor contractility with ageing.