The Wnt gene family encodes a group of proteins probably involved in cell-cell communication during several stages of vertebrate development. More than 10 members of this family have been identified and shown to be expressed mainly in developing neural tissue. Using a reverse transcription-polymerase chain reaction (RT-PCR)-based approach with degenerate oligonucleotides directed against conserved sequences in the Wnt genes, Wnt-2 transcripts were detected in RNA isolated from mammary glands of 4- to 6-week-old virgin C3H mice, a period characterized by extensive end bud and ductal proliferation. The spatial and temporal expression of Wnt-2 in the developing mouse mammary gland was studied by in situ hybridization, quantitative RT-PCR, and Northern analysis. Wnt-2 is expressed during the ductal phase of mammary gland development, primarily in the basal layer of mammary ducts and in the body cells of end buds. Wnt-2 RNA transcripts were readily detected in poly(A) RNA isolated from 5-week-old C3H and Balb/c mice. RNA transcript levels measured as molecules per nanogram of total RNA by RT-PCR decreased 10- to 40-fold within 2 days after the onset of pregnancy and remained low during pregnancy and lactation. This is in contrast to the patterns of expression of other Wnt family members, Wnt-5a and -5b, whose expression was either barely or not detectable in the 4- to 6-week-old mammary gland, but increased markedly during pregnancy. These results confirm the differential expression of Wnt gene family members during mammary gland development. Furthermore, they suggest that Wnt-2, as well as several other family members, may play a role in pattern formation during early mammary gland development.