The blasts, which caused no injuries, came on Friday, a day after Spanish politicians expressed hope that the Irish Republican Army's announcement that it was laying down arms might encourage ETA to do likewise.
A police spokesman in the Castilla La Mancha regional capital of Toledo said the two bombs exploded on roads in central Spain after a warning call in the name of ETA to the pro-Basque independence newspaper Gara. ETA regularly warns of its attacks through calls to Gara.
Gara said the anonymous caller had given the location and timing of the blasts.
The first blast occurred at 6pm (1600GMT) about 75km (46 miles) southwest of Madrid on the A5 motorway.
A second device exploded 140km south of Madrid along the A4 15 minutes later.
News reports said both devices exploded on the shoulder of the roads causing little damage.
The blasts happened as millions of Spaniards took to the roads to begin their summer holidays. Traffic on both highways was interrupted by the blasts.
The blasts came hours after the Spanish Interior Ministry announced that French police had arrested four suspected ETA members in raids on Thursday.
Also on Friday, two former leaders of the armed Basque separatist group were sentenced to 32 years in prison for ordering the kidnapping of a prison official in 1996.
ETA, an acronym for Basque Homeland and Freedom in the Basque language, has killed more than 800 people since it began a campaign in the 1960s for an independent state.
Its last lethal attack was in May 2003 when it killed two police officers with a car bomb.
Police have repeatedly said they believe the group has been decimated by arrests in Spain and France in recent years.