The effect of various neurotransmitters and some of their agonists and antagonists on the crayfish abdominal positioning system.

1. Crayfish abdominal nerve cords were perfused with selected transmitters or their agonists or antagonists. Motor activity underlying abdominal positioning behavior was monitored. 2. All the neurotransmitters except glycine had a measurable effect on this system. 3. Acetylcholine and its agonists were slightly stimulatory. Both muscarinic and nicotinic receptors were indicated. 4. GABA was weakly inhibitory. Picrotoxin was strongly stimulatory, perhaps as a result of its known ability to block GABA and inhibitory acetylcholine receptors. 5. Histamine was strongly inhibitory. Both H1 and H2 receptors were indicated. 6. Glutamate was found to be slightly inhibitory while its agonist, NMDA, showed no effect. 7. Finally, L-Dopa was stimulatory, but only at a high concentration.