A single application of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) to hairless mouse skin induces increased activity of epidermal L-ornithine carboxy-lyase (E.C.4.1.1.17) (ODC) with a peak at 5 h, and S-adenosyl-L-methionine carboxy-lyase (E.C.4.1.1.50) (SAM-D) with a broad peak at 20-36 h. The temporal sequence of the accumulation of polyamines; i.e. putrescine, spermidine and spermine, and the rate of DNA synthesis was investigated. All four parameters were measured in the same tissue-samples and multiple peaks of DNA synthesis and of individual polyamines were demonstrated. In the period from 0-12 h, there was an initial decrease in the rate of DNA synthesis. In this period changes in the molar ratio of spermidine/spermine were negatively correlated to the rate of DNA synthesis. From 12-48 h, however, changes in the molar ratio of spermidine/spermine had an almost identical time course with rates of change of DNA synthesis. Based on corresponding cell kinetic results it is suggested that the spermidine/spermine ratio reaches a maximum peak during the S-phase of the cell cycle. The relation between the rate of DNA-synthesis and the spermidine/spermine ratio as well as the ordered time sequence for the accumulation of putrescine and the induction of ODC and SAM-D activities, suggest a strong interdependence and a strict regulation of these events in hairless mouse epidermis induced to proliferate by TPA.