Photoperiod and heat load are two of the most important environmental factors influencing the reproduction of sheep, goats, pigs, cattle and equines. Seasonal variations in sexual activity among small: ruminants depend on latitude and length of day. Melatonin secreted during the night by the pineal gland enables the animals to determine the length of day. This hormone can be used to advance the mating season of sheep and goats and increase their fecundity. With regard to the males, the use of artificial photoperiodic variations and/or melatonin eliminates or alters the seasonality of sperm production. Under heat stress, the libido, sperm production and fertilization capacity of males are significantly altered. In females, heat stress reduces the oestrus period and level of fecundity and increases early embryonic mortality. These effects can be countered by keeping animals in the shade, spraying them with water and using forced ventilation.