A high throughput and high sensitive online solid phase extraction coupled to high performance liquid chromatography-tandem mass spectrometry method was established and validated for simultaneous quantification of four cobalamins including OHCbl, CNCbl, AdoCbl and MeCbl in rat plasma. Analytes were first enriched on an online SPE Shim-pack MAYI-ODS column and then separated using a Poroshell 120 EC C18 analytical column. The linear range was from 0.05 to 5 ng/ml and the LLOQ was 0.05 ng/ml for all four cobalamins. In addition, this method was applied to determine the cobalamins levels in pentylenetetrazole-induced kindling rat model for the first time.