Precursor incorporations into DNA, RNA and protein fractions of the human testicular tissue were examined at 28 degrees C-43 degrees C in vitro to study the optimal temperature for DNA, RNA and protein synthesis in the human testis. The results suggest that the DNA synthesis shows delicate temperature sensitivity and is at a maximum at 31 degrees C, whereas the RNA and protein syntheses are at a maximum at 37 degrees C-40 degrees C. The slight variation of the testicular temperature may cause spermatogenic dysfunction because of delicate temperature sensitivity of the testicular DNA synthesis.