Spectrophotometric quantification of fluid phase endocytosis in the presence of different pharmacological compounds was performed in the model unicellular eukaryote Paramecium. The kinetics of Lucifer Yellow Carbohydrazide (LY) uptake in cells exposed to forskolin and isoproterenol--known to stimulate phagocytosis in this cell--was analyzed. Reduction in both the rate of endocytosis and total accumulation of fluid phase marker was observed following the treatment. Forskolin diminished total LY accumulation by 11% and 21% after 5 min and 25 min of incubation, respectively, whereas the rate of uptake was lowered by 21% in comparison to control cells. The inhibitory effect ofisoproterenol was less pronounced than that of forskolin. The total accumulation of LY was decreased by 11% in 5 min as compared to the untreated cells and this effect was persistent upon further exposition to this reagent up to 25 min. To better understand these observations, the effect of inhibitors of PKA and cAMP phosphodiesterase on fluid phase uptake was tested. 3-isobutyl-1-methyl xanthine (IBMX) caused 12% decrease in LY accumulation after 5 min of incubation. In combination with isoproterenol or forskolin, IBMX enhanced their inhibitory effect on fluid endocytosis, which was lowered by 25% and 29%, respectively. The strongest inhibitory effect on fluid endocytosis was exerted by the 10 microM PKA inhibitor, which diminished endocytosis by 35% in 5 min. These results suggest that Paramecium fluid phase uptake may be regulated through activation of PKA, although the precise mechanism of this process has not yet been elucidated.