Guinea-pig ears were treated topically with 20 nmol of 12-O-tetradecanoyl phorbol-13-acetate, or deprived of their horny layer by nine successive strippings. At different times after the treatment, the animals were sacrificed, the epidermis removed from the ears, and the RNA from the epidermis purified and analysed by dot-blot hybridization in order to assess and determine the amount of RNA able to hybridize to each one of the 18 onc-gene DNA probes. The following probes were used: v-src, v-fps, v-yes, v-ros, v-myc, c-myc, v-erb AB, v-myb, v-mos, v-Ha-ras, v-Ki-ras, v-abl, v-fos, v-fes, v-fms, c-sis, B-lym, v-raf. At 0 h, expression of B-lym and of myc and fos is seen. ErbAB mRNA is detected between 10 min and 4 h after stripping, as well as after TPA application. B-lym mRNA is detected for up to 36 h after stripping and for up to 8 h after TPA application. C-myc mRNa is detected for up to 36 h after tape stripping, but only for the first hour after TPA application. RNA complementary to the other onc probes was not detected, and synthesis of RNA complementary to an actin DNA probe was observed for 8 h after TPA application.