We investigated whether fluid-phase endocytosis in rabbit alveolar macrophages (AM) was regulated by alterations in intracellular adenosine 3',5'-cyclic monophosphate (cAMP). Suspensions of freshly isolated AM were incubated with anionic dextrans (mol mass = 10 kDa), coupled to fluorescein isothiocyanate (FITC), at either 37 or 4°C. There was a rapid increase in AM-associated fluorescence, quantified by laser flow-cytometry and video microscopy during the first hour of incubation at 37°C, which was directly proportional to the amount of tracer present in the medium. In contrast, at 4°C, AM fluorescence was similar to autofluorescence. Incubation of AM with forskolin (50 μM) or 3-isobutyl-1-methyl xanthine (IBMX; 0.1 mM) increased their cAMP content by 67 ± 2 and 52 ± 5% (mean ± SE; n = 4) and decreased FITC-dextran uptake by 29 ± 4 and 31 ± 4% (n = 3). On the other hand, incubation of AM with 0.5 mM IBMX inhibited FITC-dextran uptake by 62 ± 4% (n = 3), without any further increase in cAMP. Incubation of AM with 0.4 mM 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (CPT-cAMP), a cell-permeable analogue of cAMP, decreased FITC-dextran uptake by 48 ± 5% (n = 6). Pulse-chase experiments showed that the rate of FITC-dextran exocytosis was not affected by cAMP. We concluded that fluid-phase endocytosis in rabbit AM is regulated by cAMP and by an additional, cAMP-independent mechanism of IBMX.