We have examined the relative transcript levels of the junB and c-jun proto-oncogenes during development of the mouse testis. junB and c-jun mRNA levels are low in total RNA from intact immature or mature testes. Dissociation of testicular cells, however, increases the levels of junB and c-jun mRNAs, with higher increases in the dissociated cells from testes of 8-day-old mice than from 17-day-old or sexually mature mice. These differences in junB and c-jun mRNA levels localize to specific cell types. In testes from 8-day-old mice, the mRNA levels for both proto-oncogenes are higher in type B spermatogonia and in the interstitial cell fraction than in type A spermatogonia. In testes of 17-day-old mice, the highest mRNA levels for both proto-oncogenes are seen in preleptotene spermatocytes and interstitial cells, with decreasing levels in leptotene/zygotene spermatocytes and prepuberal pachytene spermatocytes. junB and c-jun mRNAs are nearly undetectable in pachytene spermatocytes, round spermatids, and residual bodies/cytoplasts. The increased junB mRNA levels originate not only from the expected 2.1-kilobase transcript but from a more slowly migrating transcript of about 2.3 kilobases. RNase H analysis demonstrates that this migration change was due to an increase in mRNA polyadenylation. The low levels of junB and c-jun mRNAs in intact testes and the much higher levels in isolated cells from identical testes suggest that the disruption of cell-to-cell contact increases the amount of junB and c-jun transcripts in specific cells of the testis. Coupled with this increase, structural changes are seen with the junB mRNA.