Picosecond rotation of small polar fluorophores in the cytosol of sea urchin eggs. A new fluorescence method to measure viscosity in cell cytosol [Fushimi, K., & Verkman, A. S. (1991) J. Cell Biol. 112, 719-725] has been applied to determine fluid-phase viscosity in sea urchin eggs. Freshly harvested eggs from Lytechinus pictus were loaded with the dyes 2,7-bis(2-carboxyethyl)-5-(and-6-)carboxyfluorescein (BCECF), 6-carboxyfluorescein (6CF), fluorescein, or calcein. Fluorescence lifetimes and anisotropy decay were measured in single eggs by multiharmonic, frequency-domain microfluorometry using a 1-2-micron focused laser spot and 25x air objective. In calibration solutions consisting of glycerol in pH 8 buffered sea water, probe lifetime was single exponential and probe rotation was isotropic with a single correlation time which increased linearly with viscosity in the range 1-3.6 cP. In eggs at 22 degrees C, there were single lifetimes (in nanoseconds) of 3.6 (BCECF), 3.4 (6CF), 3.2 (fluorescein), and 3.3 (calcein). Probe rotation in eggs had two components, a fast component (in picoseconds, mean +/- SE, 10-18 eggs) of 568 +/- 39 (BCECF), 311 +/- 21 (6CF), 313 +/- 15 (fluorescein), and 516 +/- 44 (calcein) and a slow component of 10-40 ns. The fractional amplitude of the fast component, corresponding to unbound dye, was 0.72-0.81. Apparent viscosities of fluid-phase cytoplasm (centipoises) given by the four different probes were in good agreement: 2.3 +/- 0.2 (BCECF), 2.1 +/- 0.1 (6CF), 2.5 +/- 0.1 (fluorescein), and 2.3 +/- 0.2 (calcein). The viscosity in cytosol of sea urchin eggs (2.1-2.5 cP) is thus relatively low, yet significantly greater than that of water (1 cP) or cytosol in cultured fibroblasts (1.2-1.4 cP).(ABSTRACT TRUNCATED AT 250 WORDS)