We have used a modified photodiode method to measure ciliary beat frequency of human neonatal cilia, and have developed techniques to allow similar measurements of rat brain cilial function. The effect of temperature change, within the physiological range, on neonatal human respiratory cilia and neonatal rat brain cilia was studied. The mean (SD) ciliary beat frequency of neonatal respiratory cilia at body temperature was 14 (1.1) Hz and that of rat brain cilia 18.9 (1.3) Hz. Respiratory cilial beat frequency decreased by 9.1% and brain cilia by 13% on cooling to 32 degrees C compared to normal body temperature. Beat frequency increased by 8.5% in respiratory samples and by 12% in brain samples on warming to 40 degrees C compared to the frequency at normal body temperature. The Q10 of human neonatal respiratory cilia was 1.2 and that of rat brain cilia 1.3.