PDZK1, a multi-PDZ domain containing adaptor protein, interacts with various membrane proteins, including the HDL receptor scavenger receptor class B type I (SRBI). Here we show that PDZK1 controls in a tissue specific and post-transcriptional fashion the expression of SR-BI in vivo. SR-BI protein expression in PDZK1 knockout (KO) mice was reduced by 95% in the liver, 50% in the proximal intestine and not affected in steroidogenic organs (adrenal, ovary and testis). Thus, PDZK1 joins a growing list of adaptors that control tissue specific activity of cell surface receptors. Hepatic expression of SR-BII, a minor splice variant with an alternative C-terminal cytoplasmic domain, was not affected in PDZK1 KO mice, suggesting that binding of PDZK1 to SR-BI is required for controlling hepatic SR-BI expression. The loss of hepatic SR-BI was the likely cause of the elevation in plasma total and HDL cholesterol and the increase in HDL particle size in PDZK1 KO mice, phenotypes similar to those observed in SR-BI KO mice. PDZK1 KO mice differed from SR-BI KO mice in that the ratio of unesterified to total plasma cholesterol was normal, females were fertile and cholesteryl ester stores in steroidogenic organs were essentially unaffected. These differences may be due to nearly normal extrahepatic expression of SR-BI in PDZK1 KO mice. The PDZK1-dependent regulation of hepatic SR-BI, and thus lipoprotein metabolism, supports the proposal that this adaptor may represent a new target for therapeutic intervention in cardiovascular disease.