Bombesin-like peptides are present in the mammalian hypothalamus. It has previously been reported that synthetic bombesin elevates plasma growth hormone (GH) and prolactin (PRL) concentrations in rats. To investigate whether bombesin has a direct action on the pituitary gland we measured GH and PRL secretion from 5-day cultures of bovine anterior pituitary cells. We compared the effects of bombesin with those of a previously described synthetic pentapeptide which releases rat GH in vitro (GHRP) and with the effects of acetylcholine which stimulates bovine GH secretion. Bombesin stimulated GH but not PRL secretion from pituitary cultures during 90-min incubations. Maximal stimulation to 157 ± 16% of the control value was seen with 1.2 × 10<sup>-7</sup> M bombesin. Half-maximal stimulation occured with 5 × 10<sup>-9</sup> M bombesin. The pentapeptide GHRP also stimulated GH but not PRL secretion. Maximal stimulation to 207 ± 14% of the control value was seen with 1.3 × 10<sup>-5</sup> M GHRP. Half-maximal stimulation occurred with 10<sup>-7</sup> M GHRP. Acetylcholine stimulated secretion of both GH and PRL, both responses being abolished by the muscarinic antagonist atropine. Maximal stimulation with 10<sup>-4</sup> M acetylcholine was to 381 ± 33% of the control value for GH and to 134 ± 5% of the control value for PRL. The effects of bombesin and GHRP on GH secretion were not additive whilst the effects of both peptides were additive with those of acetylcholine. The data suggest that the synthetic GHRP peptide may possibly interact with native bombesin receptors on pituitary somatotrophs and that bombesin or a related peptide may act in vivo as a GH releasing factor.