The present experiments were designed to study behavioral effects of two chemicals, which have opposite influences on the cholinergic neurotransmitter system, and of their interaction. It has been proposed that deanol is a direct precursor of acetylcholine (ACh) in brain and may enhance cholinergic transmission, while hemicholinium-3 (HC-3) acts to decrease ACh synthesis. Rats served as subjects. Doses of the drugs were based on results of earlier experiments; all were injected cerebroventricularly. The six treatment groups were: saline only; HC-3 (10 micrograms); HC-3 (10 micrograms) + deanol (1 microgram); HC-3 (10 micrograms) + deanol (10 micrograms); deanol (1 microgram); and deanol (10 micrograms). Behaviors measured were: reactivity to visual and tactile stimuli; resistance to capture and handling, vocalization, muscular tension; reactivity to non-contingent aversive stimulation; and, shock-induced defence reaction. With the exception of the defence reaction, all behaviors showed significant effects between the various drug treatments: deanol had no significant effect on the behaviors; animals receiving HC-3 only clearly showed responses which were enhanced above the levels of any of the other treatment groups; deanol had a dose-dependent effect of suppressing HC-3 enhanced behavior. The present results are consistent with the generalization that decreased cholinergic activity is associated with hyper-reactivity, and increased cholinergic activity with hyporeactivity. They indicate that the behavioral effects of deanol are dependent upon the state of the cholinergic system, interacting in combination with HC-3 but not alone.