Placental transfer and fetal toxicity of pesticides have been documented in normotensive rats but no reports appear in the literature regarding the susceptibility of spontaneously hypertensive (SHR) perinates to pesticide challenge. This report describes the effects of prolonged parathion exposure to perinatal rats born to hypertensive dams. Ninety day old, spontaneously hypertensive (WKY strain) male and female rats were bred and vaginal plugs were used to determine that copulation had occurred. Parathion (0.01, 0.10 and 1.00 mg/kg) suspended in peanut oil was administered daily to the dam via oral intubation from day one of gestation to day fifteen of lactation, a total of thirty-seven days. On day twenty-four post partum the perinates were examined. Male and female SHR perinates exhibited a significant reduction in plasma cholinesterase at the 1.00 mg/kg dose level. Changes in relative organ weights, hematological parameters and serum enzymes were found. Heart rate was significantly reduced at the 0.01 mg/kg and 0.10 mg/kg dose levels. The results of this study indicate that exposure of SHR dams to parathion during gestation and lactation resulted in toxicological effects in the perinate basically similar to those seen in normotensive perinates.