Heat stress has long been recognised as a cause of subfertility in farm animals. The objectives of the present study were to elucidate the effect of heat stress on sperm function and involvement of the mitogen-activated protein kinase (MAPK) 14 signalling pathway. Spermatozoa incubated for 4h at a physiological temperature (38.5 degrees C) exhibited significantly (P<0.05) reduced motility, plasma membrane integrity and mitochondrial potential compared with non-incubated spermatozoa; the reductions in these parameters were more severe following incubation at a hyperthermic (41 degrees C) temperature (P<0.01). Percentages of fertilisation and embryo development were highly affected in spermatozoa incubated at 41 degrees C compared with non-incubated spermatozoa (P<0.01). Similarly, embryo quality was adversely affected by sperm incubation at 41 degrees C, as indicated by a higher apoptotic cell ratio in Day 7 blastocysts compared with that in the non-incubated control group (14.6% vs 6.7%, respectively; P<0.01). Using SB203580 (10 mu gmL(-1)), a specific inhibitor of the p38 MAPK pathway, during sperm hyperthermia reduced MAPK14 activation (24.9% vs 35.6%), increased sperm motility (45.8% vs 26.5%) and reduced DNA fragmentation (16.9% vs 23.4%) compared with the untreated control group, but did not improve subsequent fertilisation and embryo development. In conclusion, heat stress significantly affects the potential of spermatozoa to penetrate oocytes, as well as subsequent embryo development and quality. Notably, the data show that the MAPK14 signalling pathway is largely involved in heat-induced sperm damage. However, further research is needed to elucidate other signalling pathways possibly involved in heat-induced sperm damage.