The ontogeny of P1 purinoceptors in the separated layers of the rat duodenum was investigated using functional assays. In the longitudinal muscle N6-cyclopentyladenosine (CPA) caused relaxations from day 20 that were inhibited by 1,3-dipropyl-8-cyclopentyl-xanthine (DPCPX) (10 nM) indicating an action via adenosine A1 receptors. 5'-N-ethylcarboxamidoadenosine (NECA) caused relaxations at day 15 that were inhibited by DPCPX (1 microM) while 2-p-(2-carboxyethl)phenylethylamino-5'-N-ethylcarboxamidoade nosine (CGS 21680) was almost inactive, indicating an action at adenosine A2B receptors. From day 20 NECA was inhibited by DPCPX (10 nM) but was not antagonised by DPCPX (1 microM) to the extent expected for an adenosine A1 receptor, suggesting activation of adenosine A1 and adenosine A2B receptors. In the muscularis mucosae, CPA and NECA caused contractions from day 10 inhibited by DPCPX (1 microM) while CGS 21680 was less potent, indicating activation of adenosine A2B receptors. These results show that adenosine A2B receptors are present early in the postnatal period, whereas adenosine A1 receptors develop after day 20.