One of the most important functions of hepatocytes is the synthesis of serum proteins, more specifically of serum albumin. Albumin secretion in serum is essential, since it maintains the oncotic pressure in the body. Measurement of albumin secretion is used as a liver function test to indicate potential liver injury and liver pathology. In this chapter, a protocol for the measurement of albumin secretion in the supernatant of cultured rat hepatocytes is described. The procedure relies on an enzyme-linked immunosorbent assay allowing rat albumin to be quantitatively measured.