Tyrosine sulfation is mediated by one of two Golgi isoenzymes, called tyrosylprotein sulfotransferases (TPST-1 and TPST-2). A relatively small number of proteins are known to undergo tyrosine sulfation, including certain adhesion molecules, G-protein-coupled receptors, coagulation factors, serpins, extracellular matrix proteins, and hormones. As one approach to explore the role of these enzymes in vivo and how they might interact in biological systems, we have generated TPST-1-deficient mice by targeted disruption of the Tpst1 gene. Tpst1(+/-) mice appear normal and, when interbred, yield litters of normal size with a Mendelian genetic distribution and an equal sex distribution. Tpst1(-/-) mice appear healthy but have approximately 5% lower average body weight than Tpst1(+/+) controls. In addition, we show that although fertility of Tpst1(-/-) males and females per se is normal, Tpst1(-/-) females have significantly smaller litters because of fetal death between 8.5 and 15.5 days postcoitum. These findings suggest that there are proteins involved in regulation of body weight and reproductive physiology, which require tyrosine sulfation for optimal function that are yet to be described. Our findings also strongly support the conclusion that TPST-1 and TPST-2 have distinct biological roles that may reflect differences in their macromolecular substrate specificity.