Male Wistar rats exposed intermittently to 300 ppm (12.5 mumol/l) morpholine vapour 5 days a week for 6 h daily during 4-15 weeks displayed an increasing brain solvent content analyzed by a sensitive gas chromatographic method. Rats drinking water which contained 150 mg NaNO2/l during the vapour exposure period showed initially larger brain solvent concentrations which began to decrease after 8 weeks. Fat solvent concentrations were a fraction of those detected in brain of the solvent-exposed animals or of those in the combined exposure. Nitrite exposure alone caused decreased spinal cord axon acetylcholine esterase activity after 8 weeks while this effect was noted in the combined exposure only at 8 weeks disappearing later on. Axonal succinate dehydrogenase activity was below the controls in the combined exposure throughout the study, and combination also caused persistent increase in the muscle creatine kinase activity. The morpholine-induced effects were less remarkable. The results point at pharmacokinetic interaction between the solvent and nitrite with its own effects on energy metabolism. No N-nitrosomorpholine was found although other metabolic interactions could not be excluded.