The effects of optically pure d-nicotine were investigated as compared with those of the l-isomer. d-Nicotine showed qualitatively the same effects as the l-isomer in the ganglionic or neuromuscular sites and the relative potency of the d-isomer was approximately 0.06 in the case of rat blood pressure (elevation), approximately 0.2 in the cat superior cervical ganglion (stimulation and blockade) and approximately 1.0 in the neuromuscular junctions of the rat diaphragm (blockade). In the adrenergic nerve terminals of the isolated rabbit pulmonary artery, l-nicotine produced sympathomimetic effects by releasing norepinephrine from those terminals. d-Nicotine, on the other hand, did not produce such effects, but instead inhibited those effects due to the l-isomer. Because d-nicotine had no effect on the response to exogenously applied norepinephrine and blocked the 3H-efflux induced by the l-isomer from the preparations preincubated with [3H]norepinephrine, the inhibitory effect of the d-isomer was attributed to its presynaptic action. Such an inhibitory effect of d-isomer was noncompetitive, on the basis of the shift of the concentration-contraction curve with the l-isomer. Simultaneous application of both isomers produced no inhibition of the response to the l-isomer, irrespective of the concentration of d-nicotine. Occurrence of the inhibitory effect of d-nicotine was not prevented by hexamethonium. d-Nicotine did not inhibit the responses of the artery to electrical transmural stimulation. These results indicate that d-nicotine inhibits the response to l-isomer by acting neither on the nicotinic receptors nor on excitation-secretion coupling mechanisms. Furthermore, such an effect of the d-isomer would be similar to that reported in the case of l-isomer, i.e., "nicotinic desensitization."