AbstractProtein synthesis and secretion in mouse uterine glands during the peri‐implantation period were studied, by both light and electron microscopic autoradiography, after the in vivo administration of tritiated leucine (3H‐leucine) and proline (3H‐proline). Light microscopic autoradiography revealed that the time course of synthesis and secretion of labeled proteins was constant during days four, five, and six of pregnancy. Labeled material could be detected in the glandular lumen by 45 minutes after administration and in higher concentrations by 90 minutes after administration.Analysis of electron microscopic autoradiographs from days five and six of pregnancy showed that high levels of activity were initially present over the rough endoplasmic reticulum and Golgi complexes and subsequently declined at the longer time intervals (45 and 90 minutes), while activity over the glandular lumen increased with time. The pathway of intracellular transport to the glandular lumen appeared to be via small cytoplasmic vesicles on both days five and six of pregnancy. Additional pathways for transport of the labeled protein to the glandular lumen appeared to be present in the form of the large vesicles on day five and granules on day six of pregnancy.Throughout the peri‐implantation period, mouse uterine glands were active secretory structures in which the mode of secretion was similar to other exocrine cells. Thus, the uterine glands of the mouse must be considered a source of uterine fluid proteins at the time of implantation that may contribute to quantitative changes in these p