Thin slices of s.c. implanted B-16 melanomas as well as of human melanomas have been incubated for 5 h with (H3) Uridine and (H3) Thymidine in the presence of different chemotherapeutical agents, whose concentration was equivalent to the tenfold therapeutical daily dose in men. In this short term test model, the sensitivity of a melanoma to a chemotherapeutical agent is indicated by the inhibition of the nucleoside uptake by more than 50%. The in vitro sensitivity rates, each based on 10--30 melanomas, are compared to the in vivo sensitivity rates. Sensitivity is indicated by the increase of life span (greater than 25%) in the melanoma bearing mice respectively by the regression of human melanoma metastases (greater than 50%). -- The in vivo sensitivity of the B-16 melanomas, evaluated by the uridine and/or thymidine uptake, was in line with the in vivo sensitivity to all chemotherapeutical agents with the exeption of Adriamycine and DTIC. The in vitro sensitivity of human melanomas to Dactinomicine, Vincristine, BCNU and DTIC corresponds to the in vivo sensitivity whereas no in vitro/in vivo correspondence could be observed in testing Bleomycine, Procarbacine and 5-Fluorouracile. Comparing the sensitivity of B-16 and human melanomas, similarity was observed in vitro but not in vivo.