The blood-testis barrier (BTB) is a permeability barrier vital to spermatogenesis. It functions to seclude meiotic spermatocytes and differentiating spermatids from contact with the vascular environment. However, our lab has shown that small molecules (<0.5 kDa) can permeate the BTB when spermatocytes are present¹. Therefore, we hypothesised that meiosis is not dependent on an impermeable BTB, and aimed to characterise BTB permeability during meiosis, using tracers that emulate the size of blood- and lymphatic-borne factors that could breach the BTB. Utilising a model of meiotic suppression and re-initiation, adult rats (n=9/group) received the GnRH antagonist acyline for 10 weeks, alone, or with subsequent administration of testosterone (T24cm Silastic implant), for 7, 10, 15 and 35 days. Small, medium and large (0.5, 70 & 150 kDa) tracers were delivered by intratesticular injection and testes were fixed by lower body perfusions with Bouin’s solution. The extent of tracer permeation was assessed using confocal immunofluorescence, and correlated to all stages of spermatogenesis using stereology. In untreated testes, the 0.5 and 70 kDa tracers, but not 150 kDa, permeated around spermatocytes up to the leptotene stage XI. In suppressed testes, all tracers permeated throughout the seminiferous epithelium. After 7 days of testosterone, all pachytene spermatocytes up to the diplotene stage XIII reappeared, and both the 0.5 and 70 kDa tracers, but not 150 kDa, permeated around these cells. However, intriguingly all tracers were excluded from pachytene spermatocytes at stages VII and VIII, whereas spermatocytes in stages IX-XIV were again permeable to the 0.5 and 70 kDa tracers. The BTB became impermeable to the 0.5 and 70 kDa tracers when stage VII round spermatids first appeared after 15 days of testosterone. In summary, blood- and lymphatic-borne factors up to 70 kDa may not be detrimental to meiosis. Larger factors (≥150 kDa), including antibodies, are excluded from contact with all spermatocytes. An additional level of BTB sequestration occurs around stages VII and VIII pachytene spermatocytes when spermatogenesis is restarting. We conclude that meiosis can occur with a partly functional BTB. This research has important implications for understanding the effects of BTB dynamics on spermatogenesis, particularly in male hormonal contraceptive development, which uses a similar suppression paradigm.