The hypothesis was tested that accumulation of osmolytes by kidney cells grown in hyperosmolar media decreases the rotational and translational mobilities of small polar solutes in the cytosolic compartment. Rotational mobility was measured by the picosecond rotational correlation times (tau c) of 2',7'-bis(2-carboxyethyl)-5(6)carboxylfluorescein (BCECF) by multiharmonic microfluorimetry. In isolated segments of rabbit proximal tubule, thick ascending limb, and cortical collecting duct that were perfused and bathed in 300 mosM media, tau c were in the range 180-250 ps, corresponding to apparent rotational viscosities (eta r) of 1.1-1.5 cP. In cortical collecting tubule, eta r was not influenced by serosal vasopressin. In Madin-Darby canine kidney (MDCK) cells grown in 300-1,200 mosM media, eta r increased progressively by up to a factor of 1.38 +/- 0.03; measurements of tau c and macroscopic viscosity in artificial solutions containing osmolytes supported the hypothesis that the increased eta r was due to accumulation of organic osmolytes. BCECF translational mobility was measured by fluorescence photobleaching recovery using a focused 1.2-microns diameter Ar laser beam at 488 nm. Recovery half-times were 36 +/- 3 (SE) ms (n = 10) in MDCK cells grown in 300 mosM media and 62 +/- 3 ms (n = 10) when grown in 1,200 mosM media. The results suggest that accumulation of osmolytes by renal cells is associated with significantly increased cytosolic viscosity. The increased viscosity would slow enzymatic and transport processes in the cytosolic compartment.