N-methyl-n-pyrrolidinone (CAS No. 872-50-4) was administered to groups of CD rats (15 males/ 15 females) by inhalation at 0.088, 0.423 or 0.74 mg/L for six hours/day for four weeks with 5 treatments/week followed by 14 days of recovery, although due to excessive mortality the high exposure group was taken off treatment after only 10 treatments. All groups showed concentration related increases in lethargy and irregular respiration. The high exposure group showed varying degrees of lymphoid atrophy and necrosis and adverse effects on the hematopoietic tissue, although some of the hematopoietic effects appeared to be reversible during the recovery period. The two lower exposure groups did not exhibit any abnormalities in hematopoietic organs. The high concentration group also showed evidence of testicular atrophy, and an increased neutrophil/lymphocyte ratio which reverted to normal in the recovery period. Organ weight changes which occurred at the different exposures did not have a clear treatment-relationship and no gross or microscopic findings and therefore were not considered biologically significant by the authors.