The chick embryo is a useful model for studying hematogenous metastasis. Cancer cells injected into veins of the chorioallantoic membrane (CAM) circulate briefly through all tissues but form metastases predominantly in the CAM. This respiratory organ is particularly suitable for intravital microscope because of its accessibility without the need for surgery and the density and planar configuration of its vessels (which we confirmed by microcorrosion casting). Using an inverted microscope with oblique transillumination for high-resolution images and epifluorescence to identify labeled B16F1 melanoma cells, we studied successive stages of metastasis formation in the CAM in vivo. By 2 min postinjection (pi) all cancer cells had become arrested within the microvasculature. This initial arrest appeared to be due to size restriction, based on measurements of cell and vessel diameters. At 15-60 min pi, trapped cells were seen in tapering arterioles (27%), orifices from arterioles to the capillary plexus (61%), or in the plexus itself (12%). Some cells had extravasated into the underlying mesenchyme by 3 hr (pi), and at 24 hr all cancer cells had completed this process. The mean rate of migration out of capillary lumens was approximately 1 micron/hr. Micrometastases grew in a planar configuration just beneath the capillary plexus, with a cell doubling time of approximately 24 hr. Our technique is also applicable to other tumor types and host animals and provides a powerful tool to complement studies on the molecular basis of metastasis.