The forced oscillation technique is more and more often used in the evaluation of the results of bronchomotor tests and in assessing respiratory mechanics in children. There is little information available concerning diurnal variability of indices of respiratory mechanics derived from input impedance (Z(rsin)). In 29 asthmatic children, aged 8-13 years we have studied diurnal changes of respiratory system resistance (R(rs)) and compliance (C(rs)) by measuring input impedance with the head generator every hour from 08:00 to 21:00 h. Diurnal coefficient of variation for zero frequency resistance (R(0)), mean resistance in the range 4-32 Hz (R(avg)) and C(rs) were 10.6+/-4.9, 11.2+/-4.5 and 8.9+/-3.3%, respectively. Significant circadian rhythm on R(0) was detected in 11 children, on R(avg) in nine children and on C(rs) in seven children. Only three children had all the three rhythms significant. We conclude, that diurnal variability of input impedance derived indices of respiratory mechanics is rather small, and circadian variability is present in about one third of the examined children. Diurnal variations of respiratory mechanics in children with significant circadian rhythm do not exceed 20%.