A migrant was found hiding in a vehicle's glove box on the border between Morocco and the Spanish enclave of Melilla.
He was one of four migrants found inside four separate vehicles in the space of several hours on Saturday morning, say authorities.
Two males, ages 20 and 21, had to be stabilised by doctors after they were found to be suffering from asphyxiation, disorientation and joint pain, due to the conditions in which they travelled.
One migrant, a 20-year-old, had been found in the bottom of a lorry and had been in danger of "being caught in the axles".
A 15-year-old girl was also among those found.
Spain's Civil Guard said the hiding places — which included behind the dashboard and between the rear seats and the boot — posed a "serious risk" to the migrants' health.
Speaking to Euronews, the press spokesperson for the Civil Guard said the four people were discovered in "very reduced compartments" and were in danger of "breathing toxic gases".
The spokesperson added that an investigation was underway to determine whether the four migrants were related as it was believed that they had all travelled from Conakry, Guinea.
Three of the drivers have since been arrested - the fourth was released as the stowaway had not needed help to fit into their hiding place, unlike the others.