We compared the effects of unselective cholecystokinin (CCK) agonists (caerulein and CCK-8s) and a CCKB agonist CCK-4 on the secretion of thyrotropin (TSH), growth hormone (GH) and prolactin (PRL) in male rats. The subcutaneous (s.c.) administration of caerulein and CCK-8s suppressed dose-dependently TSH and GH levels. In contrast, when given into the 3rd brain ventricle (i.c.v.) caerulein dose-dependently elevated the GH levels. Next the importance of the afferent vagal nerves was studied in the action of caerulein and CCK-4. Subdiaphragmatic vagotomy itself decreased cold-stimulated TSH levels but abolished the suppressing effect of intraperitoneal (i.p.), and apparently also that of the i.c.v. caerulein. GH and PRL levels were altered neither by vagotomy nor caerulein. CCK-4 did not affect hormone levels. Atropine and butylscopolamine (i.p.) themselves did not alter TSH, PRL or GH secretion in intact rats. Neither did they reverse the effect of caerulein on TSH. In conclusion, CCKA receptors dominate in TSH and CCKB receptors in GH regulation. CCKA receptors in the gastrointestinal tract, related to the nervus vagus are mediating the inhibitory effect of caerulein upon TSH secretion but inhibition of GH secretion does not depend on the nervus vagus. CCKB receptors in the brain stem or near the 3rd brain ventricle are responsible for stimulation of GH secretion.