We have investigated the sexual dimorphism of the mouse major urinary proteins (MUPs) by isoelectric focusing (IEF). In each of two inbred strains which have different male patterns (C57BL and BALB/c), the females show a simpler pattern with fewer prominent components. The main female component is different in each strain, and these may be the products of allelic structural genes. Treatment with testosterone induces the excretion of MUPs with the male pattern. The in vitro translation of hybrid-selected MUP mRNA was studied in the same way. Again, the female patterns were simpler than the male, and the patterns obtained with female mRNA from the two inbred strains were different. When the females were treated with testosterone, a male or male-like pattern was obtained. We argue that some MUP structural genes are quite actively transcribed in females and that the transcription of others is dependent to different degrees upon induction by testosterone.