As the U.S. Supreme Court weighs Baze v. Rees and the constitutionality of Kentucky's -- and, by extension, nearly every other state's -- protocol for executing inmates via lethal injection, you might want to revisit "Uncomfortably Numb," Malcolm Gay's 2004 in-depth story about Missouri's death-penalty procedure.
The three chemicals are sodium pentothal, a short-acting barbiturate; pancuronium bromide, a muscle relaxant commonly used to immobilize patients during surgical procedures; and potassium chloride, which causes cardiac arrest. All three chemicals are administered individually, each in a theoretically lethal dose.
The legal argument zeroes in on the fact that executions are seldom carried out by trained medical practitioners (think of the Hippocratic Oath), and also on pancuronium bromide. If anything were to go awry during the first injection of sodium pentothal, an inmate would render an inmate fully conscious and susceptible to pain -- but completely paralyzed and unable to convey his agony.
Here are links to Gay's lethal-injection stories: