WASHINGTON, Jan. 16 (RNS) -- Evangelist Franklin Graham will take the place of his father, Billy Graham, and give the invocation at the swearing-in of President-elect George W. Bush because the elder Graham is ill.

A. Larry Ross, Billy Graham's spokesman, said Tuesday the elder evangelist is undergoing medical tests and awaiting surgery to continue treatment for the buildup of fluid on his brain.

"Tests that were conducted last week and over the weekend revealed that the shunt inserted this summer to treat normal pressure hydrocephalus is not functioning properly," Ross said.

Graham has had a role in eight presidential inaugural events, including giving the invocation and benediction at the 1989 inaugural ceremony of Bush's father, George Herbert Walker Bush.

"I consider it a great privilege to have been invited by President-elect Bush to participate in his inaugural ceremony," Billy Graham said in a statement. "I love and respect his entire family very much, and regret that I am not able to join them for this special occasion."

In addition to giving the invocation at the swearing-in on Saturday, Franklin Graham also will preach the sermon at the inaugural prayer service set for Sunday at the Washington National Cathedral.

"He's honored and excited about it," said Mark DeMoss, Franklin Graham's spokesman. "It's a great opportunity, a rare opportunity, and is something he's taking very seriously. He pretty much cleared his schedule this week to work on both of these."

Ross said the elder Graham had been invited to speak at the cathedral service but had declined "to conserve his energy." Graham had hoped to sit with the Bush family during the service.

"Obviously, of course, he's disappointed that he can't be in Washington on this special occasion, not only for our nation, but for the Bush family with whom he's had a warm friendship for many years," Ross said. "At the same time, he and his wife Ruth ... are very proud that their son Franklin can offer the inaugural prayer in his place."

The Rev. Kirbyjon Caldwell, an African-American pastor who leads Windsor Village United Methodist Church in Houston, is scheduled to give the benediction at the swearing-in and is among the list of participants from a variety of faiths at the cathedral service.

Other scheduled participants at the service include Greek Orthodox Archbishop Demetrios; Roman Catholic Archbishop Theodore McCarrick of Washington; Pentecostal minister Jack Hayford, founding pastor of the Church on the Way in Los Angeles; and contemporary Christian singer Michael W. Smith.