The functional and morphological evidence for the blood-testis barrier is discussed, together with evidence for the various processes (simple diffusion and facilitated diffusion) by which various substances enter the seminiferous tubule. Data are presented to show that methylmethane-sulfonate (MMS) and dimethylnitrosamine (DMNA) both enter the seminiferous tubules rapidly, although from the published rates of methylation of testicular DNA, by these two compounds, it might be expected that the entry of DMNA would be slower than that of MMS. It appears, however, that DMNA in blood is gradually converted to some nonpermeant compound. The possibility, as yet unsubstantiated, is discussed that a nontoxic permeant precursor may be converted into a nonpermeant toxic substance inside the tubules, thereby effectively concentrating the toxic compound inside the tubules.