Serotonin (5HT) stimulates phosphoinositide turnover in a number of tissues, but it is not known whether this effect is due to activation of a 5HT receptor which is coupled to phosphoinositide hydrolysis or if the effect is secondary to 5HT stimulated arachidonate metabolism or to the release of another neurotransmitter. In the present study we show that neither indomethacin nor BW 755C inhibits 5HT stimulated phosphoinositide hydrolysis in rat cerebral cortex, suggesting that neither cyclooxygenase nor lipoxygenase activity is required for the response to 5HT. Proteinase inhibitors do not potentiate the response to 5HT, suggesting that 5HT's effect is not due to stimulation of release of a peptide neurotransmitter. Tetrodotoxin does not inhibit the effect of 5HT and 5HT's effect is additive with that of KCl and veratrine. These data suggest that 5HT stimulated phosphoinositide hydrolysis is not dependent upon release of another neurotransmitter.