Haematocrit values in function of time were studied in normal subjects and chronic cardiopathoic and bronchopneumopathic patients in clinical steady-state not receiving treatment. No significant changes were noted over the period 8-12 a.m., during the circadian (12hr) cycle, or at 8 a.m. each day for 5 consecutive days in normal subjects. While equally insignificant, variations were more marked in the diseased patients.