The acute toxicity and primary irritancy of the industrial chemicals 5-ethylidene-2-norbornene (ENB) and 5-vinyl-2-norbornene (VNB) were studied. They are of moderate acute peroral toxicity in the rat, with LD 50 values for ENB of 2.54 (male) and 5.66 (female) ml kg -1 , and for VNB of 5.90 (male) and 11.9 (female) ml kg -1 . Percutaneous toxicity is slight in the rabbit by 24-h occluded contact, with no mortalities for ENB up to 8.0 ml kg -1 and only one mortality (male) at 16.0 ml kg -1 VNB. Dynamically generated saturated vapor atmosphere LT 50 values for ENB in the rat were 75 (male) and 125 (female) min, and for VNB they were 28 (male) and 37 (female) min. The 4-h LC 50 values for ENB were 2717 (male) and 3015 (female) ppm, and for VNB they were 2231 (male) and 2518 (female) ppm. Intravenously, the ENB LD 50 ranged from 0.09 (male rabbit) to 0.11 ml kg -1 (female); corresponding LD 50 values for VNB were 0.10-0.05 mg kg -1 . Acute neurotoxic signs were seen by the intravenous and inhalation routes of exposure, including tremors, ataxia and convulsions; the latter were sufficient to cause vertebral column luxation or fracture, producing spinal cord compression and resultant hindlimb paralysis. Both ENB and VNB are moderately irritating to the skin (rabbit), causing erythema and edema, but not necrosis. Both materials cause slight conjunctival hyperemia and chemosis in rabbits, but not corneal injury.