Expression of the ret proto-oncogene (proto-ret) in rat conceptus tissues during development was examined by in situ hybridization using photobiotin-labeled oligodeoxyribonucleic acid probes corresponding to regions coding for the kinase and transmembrane domains of proto-ret gene product. High levels of the proto-ret transcripts were detected in the cytotrophoblasts in the placenta in the mid-gestational period (days 10 and 11), but on day 14 of gestation, when the placenta was undergoing morphological changes, transcripts could no longer be detected in the trophoblasts. These results suggest that the increased expression of proto-ret may be associated with the proliferation and/or differentiation of trophoblast cells at a specific stage. Improvements in the in situ hybridization technique by introduction of an ultrafast microwave energy fixation method, and repeated-reaction cycling of avidin-alkaline phosphatase and a biotinylated anti-avidin antibody for signal amplification, are also briefly discussed.