The aim of the study was to examine the effects of insulin-like growth factor I (IGF-I) on ram sperm traits after hypothermic storage. Sperm ejaculates from Lacaune rams were diluted in a Tris extender, pooled, divided into groups of IGF-I doses tested (0, 10, 100 or 200 ng.ml−1) and stored (0–5°C) for 96 h. IGF-I elevated whole sperm motility as measured by a Computer-assisted Sperm Analyser (CASA) system, by 24 h (10 ng.ml−1) and 48 h (200 ng.ml−1) of storage, and by progressive movement on each day of storage. After 72 h the sperm samples were analysed for plasma membrane integrity (peanut agglutinin–fluorescein isothiocyanate), membrane stability (annexin V–Fluos) and apoptosis (Yo-Pro®-1) using fluorescence microscopy. The addition of IGF-I (at 100 or 200 ng.ml−1) reduced the ratio of sperm with disrupted membranes and the ratio of annexin V-labelled sperm. The ratio of apoptotic sperm was reduced by IGF-I given at 10 or 100 ng.ml−1 compared with control. Sperm fertilizing ability, determined at 48 h by an in vitro fertilization (IVF) test on bovine oocytes, was increased by IGF-I given at 100 ng.ml−1 from 47.0 to 67.7%. In conclusion, IGF-I maintained ram sperm functions following cooling storage and its effects were reflected in sperm fertilizing ability in vitro.