To assess the characteristics of sucrose as a pain-reducing substance, crying in 72 newborn humans during and after blood collection via heel prick was determined. In the first study infants drank 2 ml of water or 2 ml of a 0.17-0.34- or 0.51-M sucrose solution 1 min prior to blood collection. In the second experiment, a delay of 30, 60, 90, 120 or 240 s was imposed between sucrose intake and the initiation of blood collection. The dose-response function for concentration was flat. The most effective time delay was 120 s. The effectiveness of the 2-min interval accords with previous findings of endogenous opioid release caused by sucrose taste. The flat dose-response function extends findings in rats and humans that the calming and pain-reducing effects of sucrose are not influenced by either concentration or volume, suggesting that the transduction from gustatory afferent to opioid-mediated efferent is of an on-off nature and not graded.