Dopamine is often used clinically for the treatment of patients with shock. In a previous study, the Na-KATPase (K-NPPase) activity of strial marginal cells was inhibited after the repeated in vivo administration of dopamine hydrochloride. In the present study, the K-NPPase activity of strial marginal cells was determined using a cerium-based cytochemistry after an in vitro incubation with dopamine. The enzyme reaction product was found in untreated normal strial marginal cells, but it was almost completely undetectable after an in vitro treatment with 10 mM dopamine. This finding suggested that dopamine directly inhibited the Na-KATPase activity of strial marginal cells, and that these cells might have a dopamine receptor.