A quantitative study was made of the action of GABA, some structurally-related agonists and antagonists on the dactyl opener muscle fibres of the lobster. It was concluded that the GABA dose/conductance relationship was better described by a two independent binding-site receptor model (with KII = 30 microM) than by a single-site or a two-site high co-operativity model. The dose/conductance curves for gamma-amino-beta-hydroxybutyric acid (GABOB), delta-aminovaleric acid (DAV) and piperazine indicated 'full' agonist behaviour, whereas those for guanidoacetic acid (GuAc) indicated a partial agonist action. beta-guanidinopropionic acid (beta-GP) and gamma-guanidinobutyric acid (gamma-GB) behaved as weak competitive GABA antagonists. Bicuculline was found to antagonize GABA non-competitively on the lobster as in the crayfish, whereas picrotoxin appeared to act in a 'mixed' antagonistic fashion.