The aim of this study was to assess the concentration of isradipine in maternal and fetal plasma, and in amniotic fluid under steady-state conditions. Eight women were treated with 5-mg isradipine tablets twice daily and eight women were given slow-release isradipine capsules (SRO) twice daily for hypertension in pregnancy. Blood and amniotic fluid sampling for analysis of drug concentration was performed at delivery. In the isradipine tablet group, maternal and fetal plasma levels were 788 +/- 701 pg/mL (mean +/- SD) and 270 +/- 90 pg/mL, respectively. The corresponding levels in the SRO-treated group were 463 +/- 217 pg/mL and 185 +/- 95 pg/mL, respectively. In the amniotic fluid, the concentration was 74 +/- 42 pg/mL in the tablet group and 45 +/- 14 pg/mL in the SRO group. Therefore, isradipine passes the placental barrier, but its concentration is considerably lower in the fetal compartments.