In the process of transforming bovine milk into cheese, the main by-product is whey (WH), a by-product rich in highly digestible organic matter, but it becomes a contaminating agent for water and soil when discarded. It is estimated that the transformation of milk into cheese in the upper basin of the Ayacucho Region-Peru results in large volumes of WH dumped into rivers and surrounding soils. The present study aimed to evaluate the use of WH in guinea pig feeding. In total, 60 weaned male guinea pigs of 3 weeks of age and average weight of 310 g (298-340 g) were used. The guinea pigs were fed fresh forage (Medicago sativa) equivalent to 10% of body weight plus commercial concentrate ad libitum. There were 4 treatments with 3 repetitions and 5 guinea pigs housed per cage as experimental units: T1 100% drinking water (AP); T2 67% AP + 33% WH; T3 33% AP + 67% WH; T4 100% WH. During the exp...