Secretagogue-induced saliva of the tick, Amblyomma americanum (L.) was fractionated by reversed-phase-high-performance liquid chromatography (RP-HPLC) and bioassayed in smooth muscle preparations. Material with retention times of authentic prostaglandin E2 (PGE2) and prostaglandin F2 alpha (PGF2 alpha) were found to cause contraction of preparations of rat colon and rat stomach strips. Gas chromatography-mass spectra of selected ions of both HPLC-purified fractions confirmed the existence of PGE2 and PGF2 alpha. Bioassay of individual samples obtained from ticks stimulated to salivate with pilocarpine, dopamine + theophiline, or dopamine + theophiline + GABA indicated that all these secretagogues induced similar amounts of prostaglandin secretion, averaging 469 ng PGE2/ml. These pharmacological doses of prostaglandin are hypothesized to assist in tick feeding by inducing vasodilation and/or other pharmacological events in their hosts.