Serum albumin secretion from rat hepatocytes proceeds via the constitutive pathway. While much is known about the role of protein tyrosine phosphorylation in regulated secretion nothing is known about its function in the constitutive process. Here we show that albumin secretion is inhibited by the tyrosine kinase inhibitor genistein but relatively insensitive to sub-type selective inhibitors or treatments. Secretion is also blocked in a physiologically identical manner by the tyrosine phosphatase inhibitors pervanadate and BpV(Phen). Inhibition of either the kinase(s) or phosphatase(s) leads to the accumulation of albumin between the trans-Golgi and the plasma membrane while the immediate precursor proalbumin builds up in a proximal compartment. The trans Golgi marker TGN38 is rapidly dispersed under conditions that inhibit tyrosine phosphatase action while the distribution of the cis-Golgi marker GM130 is insensitive to genistein or pervanadate. By using a specifically reactive biotinylation probe we detected protein tyrosine phosphatases in highly purified rat liver Golgi membranes. These membranes also contain both endogenous tyrosine kinases and their substrates, indicating that enzymes and substrates for reversible tyrosine phosphorylation are normal membrane resident components of this trafficking compartment. In the absence of perturbation of actin filaments and microtubules we conclude that reversible protein tyrosine phosphorylation in the TGN is essential for albumin secretion and propose that the constitutive secretion of albumin is in fact a regulated process.