Effects of temperature on bovine sperm acrosome reaction, oocyte maturation, hyaluronic acid production by cumulus cells and in vitro fertilization were studied. Viability and a true acrosome reaction of bovine spermatozoa were impaired at 40 degrees C. Temperatures lower than 35 degrees C did not enhance the acrosome reaction. However, viability between 30 degrees C-38 degrees C was not altered after 22 h of incubation. The optimal temperature for the acrosome reaction was 38 degrees C. Labeled glucosamine incorporation into glycosaminoglycans was not different among temperatures of 35 degrees C, 37 degrees C or 39 degrees C, whereas 41 degrees C caused a significant reduction (P less than 0.02). Temperatures ranging between 35 degrees C-39 degrees C had no deleterious effects on resumption and completion of meiosis, but at 41 degrees C the frequency of oocytes that progressed to Metaphase II was significantly reduced (P less than 0.0001). Ova matured at 39 degrees C had significantly higher rates of fertilization than at 35 degrees C, 37 degrees C, or 41 degrees C. Killed spermatozoa (control) had no effect on ovum activation at 39 degrees C. From these results it was concluded that events occurring prior to and during fertilization are temperature sensitive.