A detailed understanding of the effects of diving gases on the central nervous system will require extensive neurochemical studies. One standard tool of the neurochemist is the filtration assay used to measure the binding of neuroeffectors to receptors, for example. We describe here a system for carrying our 12 such assays in hyperbaric gaseous environments. The device fits in a small pressure chamber and consists of a syringe drive for delivering solutions to the filter units brought in turn under an injection nozzle by a conveying carousel. A modified commercial filter apparatus is held on top of a vacuum-tight collection compartment. Solution is injected into an incubation well above the filter. After stirring and incubation, filtration is performed by allowing the positive pressure of the chamber to burst a diaphragm at the bottom of the incubation well and push the solution through the filter. The binding of [3H]acetylcholine to receptor-rich membranes from Torpedo californica was studied. In one experiment a percentage receptor occupancy of 49.5+/-1.4 at 5 atm was reduced to 41 7+/-0.9 at 300 atm.