Fluorescence anisotropy and average fluorescence lifetime of diphenylhexatriene were measured in artificial lipid membrane vesicles. Within the temperature range investigated (15-52 degrees C) both parameters correlate and can be used interchangeably to measure membrane fluidity. Fluorescence anisotropy of DPH in membrane vesicles of cilia from the protozoan Paramecium tetraurelia decreased slightly from 5 to 37 degrees C, yet, no phase transition was observed. An estimated flow activation energy of approx. 2 kcal/mol indicated that the ciliary membrane is very rigid and not readily susceptible to environmental stimuli. The ciliary membrane contains two domains of different membrane fluidity as indicated by two distinct fluorescence lifetimes of diphenylhexatriene of 7.9 and 12.4 ns, respectively. Ca2+ flux into ciliary membrane vesicles of Paramecium as measured with the Ca2+ indicator dye arsenazo III showed a nonlinear temperature dependency from 5 to 35 degrees C with a minimum around 15 degrees C and increasing flux rates at higher and lower temperatures. The fraction of vesicles permeable for Ca2+ remained unaffected by temperature. The differences in temperature dependency of Ca2+ conductance and membrane fluidity indicate that the Ca2+ permeability of the ciliary membrane is a membrane property which is not directly affected by the fluidity of its lipid environment.