A method for determination of N-methylsuccinimide (MSI) and 2-hydroxy-N-methylsuccinimide (2-HMSI) in human urine and of MSI in human plasma was developed. MSI and 2-HMSI are metabolites of the widely used organic solvent N-methyl-2-pyrrolidone (NMP). MSI and 2-HMSI were purified from urine and plasma by C8 solid-phase extraction and analysed by gas chromatography-mass spectrometry in the negative-ion chemical ionisation mode. The intra-day precisions in urine were 2-6% for MSI (50 and 400 ng/ml) and 3-5% for 2-HMSI (1000 and 8000 ng/ml). For MSI in plasma it was 2% (60 and 1200 ng/ml). The between-day precisions in urine were 3-4% for MSI (100 and 1000 ng/ml) and 2-4% for 2-HMSI (10,000 and 18,000 ng/ml) and 3-4% for MSI in plasma (100 and 900 ng/ml). The recoveries from urine were 109-117% for MSI (50 and 400 ng/ml) and 81-89% for 2-HMSI (1000 and 8000 ng/ml). The recovery of MSI from plasma was 91-101% (50 and 500 ng/ml). The detection limits for MSI were 3 ng/ml in urine and 1 ng/ml in plasma and that of 2-HMSI in urine was 200 ng/ml. The method is applicable for analysis of urine and plasma samples from workers exposed to NMP.