Aim The mechanism by which the duodenum adjusts the luminal osmolality remains unclear. The aim was to compare the duodenal osmoregulation in response to different hyperosmolar solutions in Sprague‐Dawley and Dark Agouti rats and to elucidate whether cyclooxygenase‐2 inhibition affects these responses. Methods The duodenum was perfused in situ with a 700‐milliosmolar solution (NaCl alone, D‐glucose ± NaCl, D‐mannitol ± NaCl or orange juice), and the effects on the duodenal motility, mucosal permeability, luminal alkalinization, fluid flux and osmoregulation were assessed in anaesthetized rats. Results The change in net fluid flux and luminal osmolality, in response to a given hyperosmolar solution, was almost identical in control rats of both strains. In control rats, hypertonic D‐glucose–NaCl induced fluid secretion only in the presence of phlorizin, an inhibitor of SGLT1. Cyclooxygenase‐2 inhibition potentiated the hypertonicity‐induced fluid secretion and increased the osmolality‐adjusting capability in both strains, but the responses were greater in Dark Agouti rats. While cyclooxygenase‐2‐inhibited Dark Agouti rats responded to the hyperosmolar solutions with depression of motility and increased mucosal permeability, these effects were absent or smaller in the Sprague‐Dawley strain. In contrast, orange juice induced the same duodenal responses in cyclooxygenase‐2‐inhibited Dark Agouti and Sprague‐Dawley rats. Conclusion The duodenum possesses the ability to absorb fluid despite a very high luminal osmolality. Inhibition of cyclooxygenase‐2 markedly enhanced the capability of the duodenum to secrete fluid and to decrease luminal osmolality, irrespective of the hyperosmolar solution or the rat strain used, and revealed notable differences between the two strains with regard to their osmolality‐adjusting capability.