Transport of 3-O-methyl-D-[14C]glucose in vitro by round spermatids was more rapid at 34 than at 37 degrees C and more rapid at 37 than at 40 degrees C. The difference between values at 34 and 37 degrees C was reversed by changing the spermatids from 37 to 34 degrees C at various times during a total incubation of 1 h. The effect of 37 degrees C relative to 34 degrees C was exerted on Km for the transport process; Vmax was unaltered. Inhibition by 37 degrees C relative to 34 degrees C was exerted on influx; efflux was the same at both temperatures. A fraction of cells from rat testis enriched in spermatocytes but free from spermatids showed greater transport at 37 than at 34 degrees C. These results indicate that one metabolic effect of body temperature (37 degrees c) as opposed to scrotal temperature (34 degrees C) is inhibition of glucose transport into round spermatids. This may contribute to the defective spermatogenesis that can occur in abdominal testes.