OBJECTIVE: To assess micturition characteristics, bladder capacity and plasma and urine osmolarity in neonates. METHODS: Fortyfive neonates were divided into three groups according to post natal ages (Group A=～3 d, Group B=～7 d, Group C=>7 d). The time of micturition and voided volume were recorded. The residual urine volume was detected using a bladder volume instrument (BladderScan BVI 2500). Plasma and urine osmolarity were measured using the FM-6 pattern freezing point osmometer. RESULTS: ① There were significant differences (P<0.05) in the daynight voided volume and voided frequency between Group A and Groups B and C. ② The bladder emptying rates, maxium bladder capacity(MBC) and plasma urine osmolarity of Groups A, B and C were 0.80±0.07, 0.82±0.09, 0.79±0.10; 56±6, 65±14, 61±15 (ml); 285±5, 287±7, 286±8 (mOsm/L); 202±59, 185±65, 216±92 (mOsm/L) respectively and there was no significant difference between the 3 groups. ③ The multiple stepwise regression method was used to estabilish the best regression equation for predicting the MBC, which was 42±0.17X (ml) ［X=8 hours (10 Am～6 Pm) voided volume, F=15.561 (P<0.01)］. CONCLUSIONS: The bladder of normal neonates empties almost completely and its function is stable. There is no difference in the renal concentration function of neonates between day and night.