The actions of three resin acids (abietic acid, dehydroabietic acid, and 12,14-dichlorodehydroabietic acid) were investigated using synaptosomes isolated from the whole brain of the rainbow trout (Oncorhynchus mykiss). Exposure of synaptosomes to resin acids caused a concentration-dependent increase in synaptosomal free [Ca 2ฯฉ ] that was unaffected by tetrodotoxin. 12,14-Dichlorodehydroabietic acid (EC50 ฯญ 12 M) was five-to sixfold more potent as a calcium mobilizing agent compared to its nonchlorinated analog dehydroabietic acid (IC50 ฯญ 68 M) and maximum increases in synaptosomal free [Ca 2ฯฉ ] were estimated at approximately 325 nM and 260 nM, respectively. Although closer to dehydroabietic acid in potency, abietic acid was more efficacious, increasing synaptosomal free [Ca 2ฯฉ ] by 500 nM at 133 M. The requirement for extracellular Ca 2ฯฉ was greatest for dehydroabietic acid compared to abietic acid or 12,14-dichlorodehydroabietic acid; however, none of the study compounds was capable of stimulating 45 Ca 2ฯฉ uptake or reducing 45 Ca 2ฯฉ efflux from synaptosomes. For 12,14-dichlorodehydroabietic and dehydroabietic acids only, the rises in synaptosomal free [Ca 2ฯฉ ] were accompanied by significant reductions in synaptosomal adenosine triphosphate levels and increases in synaptosomal oxygen consumption. Electron microscopic studies have shown that the vesicular content of synaptosomes was dramatically reduced by treatment with all three study compounds. Our investigation suggests that resin acids have a strong calcium mobilizing action in nerve endings isolated from fish brain and chlorination increases potency. We postulate that resin acids mobilize Ca 2ฯฉ from intracellular stores to facilitate neurotransmitter release and that for dehydroabietic acid (and to a lesser extent for abietic and 12,14-dichlorodehydroabietic acids), extracellular calcium is required for efficient penetration of the plasma membrane. The bioenergetic and respiratory disruption we observe is not closely linked to changes in cytosolic free [Ca 2ฯฉ ].