With eukaryotic cells, butyrate is known to induce a series of morphological and biochemical changes that mimic cellular differentiation. With platelets, we have found that butyrate (10 mmol/L) caused an approximately threefold increase in sensitivity to calcium ionophore A23187 and arachidonate. Maximum aggregation was observed at agonist concentrations of 3 mumol/L and 170 mumol/L, respectively, as compared with required concentrations of 10 mumol/L and 400 mumol/L in the absence of butyrate. Similar effects were seen with isobutyric acid, and about one-half the effect was shown with valerate and caproate, but lower homologues showed no synergistic effect. No ultrastructural changes were observed in platelets incubated with butyrate, and the aggregation effects were reversible and returned to normal on removal of butyrate. Membrane fluidity was unchanged by butyrate as measured by changes in the fluorescence depolarization of diphenylhexatriene. Butyrate caused a 60% to 70% increase in the uptake of 3H-arachidonate. Butyrate also potentiated the inhibition of platelet function by prostaglandin E1 and forskolin and uptake of 3H-forskolin was increased approximately 20%. In contrast, platelet response to other agonists (ADP, epinephrine, collagen, thrombin, and platelet-activating factor) was essentially unaffected by butyrate. These results suggest that butyrate may increase the uptake of certain hydophobic agonists and antagonists by platelets. Similar mechanisms for uptake of endogenous effectors may explain the response of eukaryotic cells to butyrate in culture.