To investigate the effects of experimentally-induced diabetes on prostatic muscarinic cholinergic receptors, the binding characteristics of [3H]quinuclidinyl benzilate ([3H]QNB) to prostatic membrane particulates were examined in four groups of rats: control, diabetic, diabetic insulin treated, and diabetic myo-inositol treated. Diabetes was induced by i.v. injection of streptozotocin (STZ), 65 mg/kg. Diabetic and diabetic myo-inositol-treated rats had hyperglycemia, hypoinsulinemia, glucosuria, polydipsia, and polyuria as well as significantly smaller prostates and lower body weights compared to control and diabetic insulin-treated animals. The densities of muscarinic receptors (Bmax) as determined by saturation studies with [3H]QNB in the prostatic plasma membranes of control, diabetic, diabetic insulin-treated and diabetic myo-inositol-treated rats were 80 +/- 8, 51 +/- 5, 78 +/- 3, and 47 +/- 7 fmol/mg of protein, respectively. [3H]QNB binding to muscarinic receptors was inhibited by muscarinic antagonists with the following rank order of Ki values: atropine much less than pirenzepine less than AF-DX 116. The pharmacological profile of the muscarinic receptors was similar in all groups examined and was consistent with the predominance of the M3 muscarinic receptor subtype in prostatic membrane particulates. Our data indicate that STZ-induced diabetes caused a variety of abnormalities including a down-regulation in the density of M3 muscarinic receptors in the rat prostate and that insulin, but not myo-inositol could prevent the development of these abnormalities.