Cell-cell interactions during embryogenesis have been shown to be important for establishing developmental fates in a number of organisms such as Drosophila and Caenorhabditis. One class of genes shown to mediate this process are receptor-protein tyrosine kinases (R-PTKs). To examine whether R-PTKs might participate in similar mechanisms operating in the mammalian embryo, an important prerequisite is to show that these genes are expressed in spatial and temporal patterns consistent with such a role. Here, we analyze the expression of eck, a member of the eph family of R-PTKs, during gastrulation and early organogenesis by in situ hybridization. eck transcripts are first detected in gastrulation stage embryos (6.5-7.5 days post coitum (dpc)) in ectodermal cells adjacent to the distal region of the primitive streak. By the neural plate stage (approximately 7.5 dpc), eck expression becomes restricted to the extreme distal end or node of the primitive streak. After the beginning of somitogenesis (approximately 8.0 dpc), eck expression persists in the node as this structure regresses toward the caudal end of the embryo. In addition, beginning at the mid head fold stage (approximately 7.75 dpc), we observe that eck exhibits a dynamic and spatially restricted expression pattern in the prospective hindbrain region. eck transcripts are initially detected in a 5-cell wide strip of mesodermal cells underlying prospective rhombomere 4 (r4). Subsequently at the beginning of somitogenesis, eck mRNA expression is observed in prospective r4. At the 4--8-somite stage, eck transcripts are observed in r4, mesenchymal cells underlying r4, and surface ectoderm in the vicinity of the developing second branchial arch. By the 10-somite stage, eck mRNA expression in these cells is downregulated. Additionally, at the 5--8-somite stage, eck transcripts are detected initially in the lateral mesenchyme immediately underlying the surface ectoderm adjacent to r5 and r6, and subsequently in surface ectoderm overlying the developing third branchial arch. These data suggest that eck may be involved in cell-cell interactions guiding early hindbrain development.