A study was made of touch domes in the lumbar skin of rats of different ages. The mean density of the domes in 11- to 29-day-old rats ("pups") was 43.1 +/- 16.6/cm2 and was approximately four times higher than that in adult animals (11.6 +/- 2.8/cm2); no loss of domes appeared to have taken place in the oldest animals examined (21 months). By repeating measurements of density in the territory of a single T13 dorsal cutaneous nerve, it was shown that density decreased during body growth because no additional domes were formed to compensate for enlargement of the skin. The mechanical thresholds of the domes were lowest in the youngest rats and highest in the oldest ones (mean values, 3.21 +/- 1.96 and 9.19 +/- 5.88 microns, respectively). The sizes of the domes increased until adulthood, the mean diameters being 170 +/- 8 and 314 +/- 9 microns, respectively in pups and mature animals. The possible implications of the findings for spatial discrimination by the rat central nervous system have been considered.