Infants and toddlers often start crying at venopuncture and the stress of crying has been known to increase the levels of plasma renin activity (PRA) and plasma aldosterone concentration (PAC), but no precise information is available. We measured the levels of PRA and PAC in blood samples taken from 30 infants and toddlers within 1 min after the onset of crying, as induced by venopuncture, and 3 and 5 min after continuation of crying (PRA1.0, PRA3.0, PRA5.0, and PAC1.0, PAC3.0, PAC5.0). The age of these subjects ranged from 1 to 30 months (median 16 months). PRA1.0, PRA3.0 and PRA5.0 were 4.0 +/- 1.8 ng/ml per hour, 5.5 +/- 2.7 ng/ml per hour, and 7.8 +/- 4.2 ng/ml per hour, respectively. PAC1.0, PAC3.0 and PAC5.0 were 210 +/- 110 pg/ml, 231 +/- 118 pg/ml and 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.