In costal ecosystems, copepods coexist with toxin-producing phytoplankton. The presence of copepods can amplify the phytoplankton toxin production and thereby increase the overall toxicity of a bloom. Copepods are not always affected by the toxins and can vector the toxins to higher trophic levels. To investigate the interactions between toxin producers and their grazers, we determined the kinetics of grazer-induced increases in toxin production and the subsequent toxin reduction in a domoic acid (DA)-producing diatom, Pseudo-nitzschia seriata. The cellular DA level of the diatom was within the range of in situ measurements. Ten days after removal of the copepods, 28% ± 8% of the cellular DA still remained in the cells. Simultaneously, we monitored the toxicokinetics of DA in two grazers; Calanus finmarchicus and Calanus glacialis. After 144 h of grazing on the toxic diet, the copepods accumulated and retained high concentrations of DA. Nine hours after exposure to the toxic diet was terminated, the copepods had depurated 70% ± 10% of the DA. The depuration lasted 4 ± 2 d and was independent of Calanus species and treatment. We explored the possible physiological responses in copepods after feeding on a purely toxic diet from gene expression profiles of C. finmarchicus. Expression of genes regulating several major metabolic and cellular processes was reduced in copepods feeding on DA-containing diatoms, and we hypothesize that this is because of exposure to DA.