While the best way to identify microalbuminuria is to determine albumin excretion rate (AER) in a 24 h urine sample. Published data have shown that calculation of an albumin/creatinine ratio (ACR) in a spot urine sample has reasonable rate of sensitivity and specificity. We aimed to evaluate the effect of daily exercise on ACR and estimate the best time for the examination of the ACR in a spot urine sample. Sixteen eligible patients with Type 1 diabetes mellitus were asked to perform varying degree of exercise periods. Urinary albumin and creatinine excretion rates during each period were determined. ACR and AER of timed urinary samples were compared with the 24 hour urinary AER. We found significant correlations between timed and 24 hour urinary AER. According to diagnostic performance tests, ACR and AER of timed urine samples were both found to be significantly more sensitive during resting period when compared with mild or moderate active periods. It is concluded that ACR and AER of a timed urine sample are sensitive and specific methods for determining microalbuminuria, while overnight resting samples give the impression of being more diagnostic.