Inflorescence, stem, and leaf samples of lettuce grown in a greenhouse in spring and autumn seasons were assayed for sesquiterpene lactones (SLs) content by high performance liquid chromatography. The concentrations of SLs were significantly higher in the inflorescences followed by upper leaf and stem compared to the other plant parts in most of the samples. SLs content (sum of lactucin and lactucopicrin) in various tissues of lettuce cultivated in spring season varied from 5.7 to 22.5 fold ranging from 27.4 ㎍/g dry weight (DW) in the upper stem (cultivar “PI 176588”) as the lowest to as high as 2,292.0 ㎍/g DW in the inflorescence (cultivar “709849-1”). During autumn cultivation, the concentration of SLs varied from 2.0 to 14.4 fold ranging from as low of 32.4 ㎍/g DW in the lower stem (cultivar “PI176588”) to as high of 838.0 ㎍/g DW in the upper leaf (cultivar “Dambaesangchu”). Higher lactucin (1.2 to 5.6 fold) and lactucopicrin (1.1 to 3.9 fold) concentration was observed during spring compared to autumn cultivation in most of the samples. SLs content in various organs of lettuce increases from the basal plant part going upwards. As lactucin and lactucopicrin are the major SLs which affects the sensory property of lettuce, their quantitative variation in the lettuce cultivars is useful for breeding new varieties with better consumer acceptance.