RNA synthesis was studied autoradiographically using [H-3]-uridine at different stages of human spermatogenesis and especially at the pachytene stage. Morphological and functional evidence have shown the X Chromosome to be allocyclic relatively to the autosomes, and apparently inactive throughout the whole meiosis. No RNA precursor was incorporated by the X at any stage of male meiosis. The authors suggest that this technique could be used in cases of human rearranged chromosomes, in order to elucidate the mechanism of spermatogenic breakdown in cases of hypofertility or sterility in humans.