The kinetics of the penetration and binding of the two commonly used antitumour drugs vinblastine and 5-fluorouracil in nonvascularized tumour tissue were studied. Multicellular human tumour spheroids (glioma U-118 MG and thyroid cancer HTh-7) were used as a model system. Radiolabelled drugs were used in all studies. To avoid disturbances in the distribution of unbound drugs a dry histological technique was used in combination with contact autoradiography. In addition, quantitative measurements of the accumulation and binding of the drugs were made. The results showed that vinblastine penetrated the spheroids less efficiently than 5-fluorouracil. Vinblastine required about 2 h to be isotropically distributed within the studied spheroids, while only a few minutes were required for 5-fluorouracil. Vinblastine seemed to be accumulated in the peripheral parts of the spheroids within 15 min. High concentrations of 5-fluorouracil, isotropically distributed in the spheroids, were observed after 2 h of incubation. Significant amounts (about half) of the accumulated drugs resisted gentle washing for 3 X 20 s plus 15 min in fresh medium. The limited penetration of vinblastine correlated well with a previously observed high resistance of spheroids to treatments of short duration with this drug.