Iran has the world's only government-regulated kidney market. We report the results of the first field study of donor behaviour in this unusual market. Participants have lower risk tolerance and higher patience levels than the Iranian average but display no difference in rationality from population averages and there is evidence of altruism among participants. We provide an examination of decision-making in extreme situations by individuals in this market, typically at the very bottom of the income distribution, and shed light on the sort of people likely to participate if other nations were to operate such markets.