Two successive ejaculates were collected from six mature Holstein bulls at 3 d intervals for 7 wks. Elevated testicular temperature was induced by complete coverage of the scrotum with insulated material for 48 h. Viability (motility and acrosome integrity) and morphological characteristics of sperm before and after thermal insult were examined. For assessment of results, collection days were grouped: days -6, -3, 0 = Period 1 (d 0 = day of testis coverage after semen collection on that day), days 3, 6, 9 = Period 2 , days 12, 15...39 = Period 3. Semen was cryopreserved on each day of collection until morphological abnormalities of sperm increased to >50%. Semen viability before and after freezing was lower in Period 3 than in Period 1 (P≤.01). These differences coincided with increased abnormal morphology. No differences in viability were observed between Period 1 and Period 2 for unfrozen semen. Once frozen, spermatozoa ejaculated during Period 2 were significantly different from Period 1 for both viability measurements, but only after 3 h incubation at 37°C (P≤.01). Mean percent pre-insult abnormal sperm level was 19.6 ± 5.7 and sperm morphology in Period 1 (pre-insult) did not differ from that in Period 2. Morphological change was first noted in Period 3 on d 12 and 15 (47.5 ± 27.4 and 65.0 ± 27.0 % abnormal sperm, respectively). Abnormal sperm peaked on d 21 (83.2 ± 22.8 %). Although bulls varied in degree and time of response post-insult, all bulls exhibited the same sequence of appearance for specific abnormalities. The sequence and peak means for these abnormalities observed over all bulls were as follows: decapitated sperm, d 15 (33.9 ± 28.8 %); diadem defect, d 18 (55.6 ± 25.8 %); pyriform heads and nuclear vacuoles (excluding diadems), d 21 (18.3 ± 17.6 and 20.8 ± 10.5 %, respectively); knobbed acrosomes, d 27 (11.6 ± 13.6 % ). Sperm morphology was followed through d 39, by which time all bulls were producing ≤50% abnormal cells (35.2 ± 8.0 %). We concluded that viability of epididymal/rete sperm was adversely affected by elevated testicular temperatures, as noted by lowered viability of cryopreserved semen, and that there is a sequence in appearance of abnormal cell types in repsponse to thermal insult of the testis. / Master of Science