Summary.-The 32p uptake in a mammary tumour induced by DMBA and in the Walker 256 carcinosarcoma was measured by external GM-tubes. The uptake was significantly higher than in the skin. During exposure to a synchronized light regime a circadian variation was present in the 32p uptake of the hormonedependent DMBA-induced tumour. The maximal 32p uptake was in the dark period, in which the highest temperature in the tumour has also been found (M0ller and Bojsen, 1975). In the hormone-independent Walker 256 carcinosarcoma there was no periodicity in 32p uptake. No variation in 32p uptake was registered in the skin of normal controls or in tumour-bearing rats.