Interaction between different neurotransmitter systems has been revealed on various objects--the myocardium, identified, intact and internally perfused isolated neurons of molluscs. In frog myocardium, the inhibitory effect of adrenaline on cholinergic response may be simulated by theophylline and cholera toxin, i.e. the substances which indirectly increase intracellular content of cAMP. Another interaction reaction--inhibition of the response to adrenaline by acetylcholine--may be reproduced by imidazole, which decreases cAMP content due to activation of phosphodiesterase. Two types of interaction between serotonin and acetylcholine were revealed in the identified neurons of the snail Helix pomatia. In some of the neurons, serotonin increases acetylcholine depolarization, whereas in other ones decreases the latter. Both the increase and the decrease of responses to acetylcholine may be reproduced by theophylline. On intracellularly perfused neurons of Lymnaea stagnalis, modulation of acetylcholine responses of cell by biogenic amines was observed, the effect being induced by the drugs added not only to the surface membrane, but to intracellular medium as well.