The effect of forskolin has been evaluated in the isolated gastric fundus from immature rats in comparison with other secretagogues. 2. Forskolin (10 nM-1 microM) caused an increase in acid production being more potent than histamine, dimaprit, bethanechol and DBcAMP and equipotent with pentagastrin and isoprenaline. 3. The response to forskolin was unaffected by ranitidine (1 microM), atropine (10 nM) and PGE1 (10 microM); conversely it was inhibited by omeprazole (3 microM). 4. Threshold concentration of forskolin (30 nM) enhanced the response to histamine, dimaprit and isoprenaline but not that to bethanechol and DBcAMP. 5. The above data confirm the strong gastric secretory activity of forskolin even in the early stage of development.