1. Normal female Wistar rats showed a wide range of baseline protein excretion and albuminuria and could be divided into two distinct groups: (a) low-to-intermediate excretors, where daily baseline albumin excretion was below 1.8 mg/24 h and (b) high excretors, where daily baseline albumin excretion was 1.8–22 mg/24 h.
2. In low-to-intermediate excretors ‘sex-associated’ urinary specific proteins, supposedly present only in male rat urine, were excreted in larger quantities than albumin which constituted only 10–15% of the total urinary protein.
3. In high excretors up to 50% of the urinary protein was albumin and other serum proteins were also present in relatively larger quantities than the urinary specific proteins.
4. Baseline albumin excretion appeared to be a good indicator of the inherent efficiency of the glomerular filter and this was reflected in the response to intraperitoneal injection of heterologous serum albumin since, during periods where no significant glomerular epithelial cell foot-process loss had been incurred, the levels of proteinuria induced correlated well with baseline albuminuria.