The kinetics of paralytic shellfish toxins in Mytilus galloprovincialis, previously exposed to Gymnodinium catenatum, was studied under depuration laboratory conditions and over a declining bloom of the dinoflagellate in the field. The variation of the levels observed throughout the laboratory experiment was characterized by a fast depuration of B1, C1 + 2, dcSTX and dcGTX2 + 3, possibly due to the gut evacuation of unassimilated toxins or microalgae cells, or loss during digestive mechanisms. Subsequent enhancements were observed for all compounds with emphasis to dcSTX and dcGTX2 + 3, pointing to biotransformation of the assimilated toxins. Then levels decreased gradually. A first-order depuration kinetic model fitted well to the decrease of B1, C1 + 2 and dcGTX2 + 3 concentrations, but not for dcSTX. Mussels exposed to a declining bloom of Gymnodinium catenatum exhibited a loss of toxins following the same pattern. Despite the low abundance of this dinoflagellate, a similar kinetic model was applied to the field data. The depuration rate of dcGTX2 + 3 in the field experiment (0.153 ± 0.03 day⁻¹) significantly exceeded the value calculated in the laboratory (0.053 ± 0.01 day⁻¹), while smaller differences were obtained for B1 (0.071 ± 0.02 and 0.048 ± 0.01 day⁻¹) and similar values for C1 + 2 (0.082 ± 0.03 and 0.080 ± 0.03 day⁻¹). The slower depuration rate of dcGTX2 + 3 in the heavily contaminated mussels at the laboratory may be related to a more effective contribution of C1 + 2 biotransformation.