Abstract The pineal gland is under complex peptidergic nervous control originating from hypothalamic nuclei. The daily rhythm of bombesin-like peptide in the hypothalamus suggests that this neuropeptide, similarly as other neuropeptides, might be involved in modulation of the physiological activity of the pineal gland. In our experiments we studied the mechanism of signal transduction of bombesin in the isolated pineal glands of rats. The phosphoinositide signalling system was examined by measuring 32P-labelling of phosphatidylinositol (PI), phosphatidylinositol phosphate (PIP) and phosphatidylinositol bisphosphate (PIP 2), which reflects phosphoinositide cycle activation. Bombesin induced a significant increase in 32P-labelling of PI, PIP and PIP 2. The antagonist of this neuropeptide, ( d-Phe 12-Leu 14)-bombesin, suppressed the increase in 32P-labelling of all phosphoinositides. Bombesin was without effect on cAMP dependent protein phosphorylation. The data indicate that bombesin activates the PI signalling system via specific receptors.