1. We have investigated the effect of diltiazem and its newly synthesized derivative (+,-)-trans-3-acetoxy-8-chloro-2,3-dihydro-5[2-diisopropylamine)ethyl]-2-(4-methoxyphenyl)-1,5-benzothiazepin-4-(5H)-ona hydrochloride (JAC-65) on several recombinant human neuronal nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes. 2. At 3 microM, both drugs have little effect on the maximal currents evoked by brief pulses of acetylcholine (ACh) in five subtypes of nAChRs (alpha7, alpha3beta2, alpha4beta2, alpha3beta4, and alpha4beta4), showing little selectivity among subtypes. 3. However, both drugs accelerate the decay of the ionic currents evoked upon continuous stimulation of ACh, being this effect larger with JAC-65, and in beta4*-nAChRs. Such an effect was dependent on the concentrations of both the drug and of the agonist used, and showed the characteristics of a non-competitive antagonism. 4. We have further investigated the effect of both drugs when combined with submicromolar concentrations of nicotine, such as those present in plasma of cigarette smokers, and found that JAC-65, but not diltiazem, is able to greatly enhance the desensitizing effect of these low concentrations of nicotine, specially in beta4*-nAChRs. 5. Experiments in alpha4beta4-nAChRs failed to show voltage dependence of the action of JAC-65. Moreover, recovery from desensitization followed the same time course regardless of the presence of the drug, suggesting that the main mechanism of action of JAC-65 does not involve open channel block. 6. In summary, both drugs, diltiazem and JAC-65, seem to act through a non-competitive mechanism, accelerating the decay of the ionic currents, being JAC-65 more effective than diltiazem at the concentrations used in beta4*-nAChRs. Thus, the differences between both benzothiazepines when measuring various parameters suggest that their mechanisms of action could be slightly different. This would require further investigation.