The levels of plasma renin activity (PRA) and plasma aldosterone concentration (PAC) have been known to increase by the influence of crying in infants and toddlers, but no precise informations are available. We evaluated the changes of PRA and PAC after crying by comparing values obtained from 30 infants and toddlers within one minute after the onset of crying, as induced by venopuncture, and three and five minutes after continuation of crying (PRA1, PRA3, PRA5, and PAC1, PAC3, PAC5). The age of these subjects ranged from one to thirty months (median sixteen months). PRA1, PRA3 and PRA5 were 4.0 +/- 1.8 ng/ml per hour, 5.5 +/- 2.7 ng/ml per hour, 7.8 +/- 4.2 ng/ml per hour, respectively. PAC1, PAC3 and PAC5 were 210 +/- 110 pg/ml, 231 +/- 118 pg/ml, 269 +/- 145 pg/ml, respectively. Both PRA and PAC increased with elapsing time. The increase in PRA was marked after a short episode of crying, but that in PAC was of a mild degree. When to evaluate the levels of PRA and PAC in infants and children, especially that of PRA, one should take into consideration of the conditions of the subject such as the duration of crying at blood sampling.