Rabbits were anesthetized with urethane and were given intracisternal injections of the following substances: adrenocorticotropin, β-melanocyte stimulating hormone, choroid plexus peptide ΠF, epinephrine, serotonin, histamine, oxytocin, lysine and arginine vasopressins, acetylcholine and melatonin. The effects on the concentration of 3’, 5’ cyclic guanosine monophosphate (cGMP) in cerebrospinal fluid were then measured. Only melatonin and acetylcholine caused a significant (p < 0.05) effect on cGMP concentration. Both agents increased the nucleotide’s concentration within 30 min. Melatonin was about 1,000 times more potent than acetylcholine; the minimal effective doses were 1 µg and 1,000 µg, respectively.