1. Pilocarpine and other cholinomimetic drugs convert isoprenaline to a vasoconstrictor and pressor agent.2. This effect of pilocarpine was abolished by atropine; it is thus an acetylcholine-like response. It was not dependent on the integrity of the central nervous system or the adrenal glands and was not abolished by ganglionic blockade.3. The constrictor action of isoprenaline after pilocarpine was abolished by propranolol; this action of isoprenaline is thus on the beta-adrenoceptor. Another beta-adrenoceptor stimulating agent, salbutamol, resembled isoprenaline in this situation, though papaverine and acetylcholine did not.4. The constrictor action of isoprenaline after pilocarpine was abolished by phenoxybenzamine, guanethidine and cocaine; the effect did not appear after reserpine pretreatment.5. These results suggest an action of cholinomimetic drugs at adrenergic nerve endings which permits the uptake of beta-adrenoceptor stimulating agents resulting in the release of neuronal transmitter.