- In the present work we report that increasing the viscosity of the medium caused not only a decrease in the ciliary beat frequency but also changes in the metachrony and correlation between cilia. The study was performed using double and triple simultaneous photoelectric measurements on cultured ciliary cells from the frog esophagus in the viscosity range of 1–2,000 cp. We observed that increasing the viscosity intensified the fluctuations in all the measured parameters. Ciliary beat frequency decreased moderately. Even at quite high viscosities (circa 2000 cp.), cilia were still active with beating frequencies of 3–5 Hz. In addition, the degree of correlation between cilia parallel to the effective stroke direction (ESD) decreased, while that perpendicular to the ESD at a low range of viscosities remained unchanged and even increased at high viscosities. Medium viscosities in the range of 30–1,500 cp. altered the metachronal wave properties of cultured frog esophagus. The metachronal wavelength increased by up to 50%, and the wave direction changed towards more orthoplectic type of coordination. According to our recently suggested model [Gheber and Priel, 1990: Cell Motil. Cytoskeleton 16:167–181], these effects can be explained by a decrease in the temporal asymmetry of the ciliary beat. Since similar results were observed in water propelling cilia of Paramecium subjected to medium viscosity ranges of up to 40 cp. [Machemer, 1972: J. Exp. Biol. 57:239–259], we conclude that hydrodynamic interactions govern the metachronal wave properties of both mucus and water propelling cilia, though mucus propelling cilia, with their better adaptation to increased load, are affected at much higher viscosities than water propelling cilia. Cell Motil. Cytoskeleton 39:9–20, 1998. © 1998 Wiley-Liss, Inc.