1. The structural requirements of an allosteric barbiturate binding site on acetylcholine receptor-rich membranes isolated from Torpedo electroplaques have been characterized by the ability of fourteen barbiturates to displace [14C]-amobarbitone binding. 2. The barbiturates could be grouped into two classes with ten barbiturates producing a strong inhibition of [14C]-amobarbitone binding (class one) and with four exerting minimal effects (class two). 3. Eight of the ten class one barbiturates displaced essentially all of the [14C]-amobarbitone from its binding site, while, at their respective aqueous solubility limits, two of these barbiturates (thiopentone and dimethylbutylbarbitone (DMBB) inhibited [14C]-amobarbitone binding by nearly 80%. The apparent inhibition constants (KI) for the class one barbiturates ranged from 13 microM for amobarbitone to 2.8 mM for barbitone with the other eight agents lying in the range 100-600 microM, and having the rank order pentobarbitone approximately secobarbitone greater than thiopentone greater than DMBB greater than butabarbitone approximately phenobarbitone greater than aprobarbitone greater than allylbarbitone. 4. By contrast, the class two barbiturates had minimal effects even at close to saturating concentrations. [14C]-amobarbitone binding was reduced slightly (less than 30%) by hexobarbitone, mephobarbitone and methohexitone and was enhanced slightly (less than 20%) by metharbitone. 5. All of the class two, but none of the class one barbiturates, were N-methylated.