Lethal and sublethal effects on juvenile Argopecten nucleus exposed to drilling muds. Offshore exploration in Colombia is in its initial stage. Currently, only few studies of acute and chronic toxicity with marine organisms evaluate drilling muds. In this paper, we test acute and chronic toxicity in juvenile organisms of Argopecten nucleus exposed to water and synthetic based muds. Aim is establish lethal median concentration (LC50) to 96 h and effects on survival and growth (height and weight) for a 30 d period. In acute tests, three water based muds (E2-LBA, E2-LBA-2 y E3-LBA-2) are below the minimal value established by EPA (1993) to be discharged in the marine environment. Given the value of the non observable effect concentration (NOEC), samples were ordered in increasing order, E3-WBM-2 < E1-SBM-2 < E1-WBM < E3-SBM-2. According to classification established by GESAMP (2002), all drilling muds evaluated from acute and chronic tests are nontoxic or with negligible toxicity, respectively. This results are the first ones in Colombia using a bivalve specie in acute and chronic toxicity tests exposed to drilling muds.