Clarified Slurry Oil (CSO), the heavy residual fraction from the fluidized catalytic cracker, was applied to the shaven backs of groups of 10 pregnant rats at doses of 0, 4, 8, 30, 125, and 250 mg/kg I day. All groups received the test material on gestation days 0–19. CSO was applied undiluted and left uncovered on the skin; collars were placed on the rats to minimize ingestion of the test material. Signs of maternal toxicity, some of which were seen at dose levels as low as 8 mg/kg/day, included vaginal bleeding, decreased body weight gain, reduced food consumption, death, increased relative liver weights, atrophy of the thymus, and aberrant serum chemistry. The number of fetal resorptions / deaths was markedly increased and the number of viable offspring decreased by CSO at dosages of 30 mg/kg/day and above. The group receiving 250 mg/kg/day carried no viable offspring. Fetuses from pregnant females exposed to CSO at dose levels in excess of 8 mg/kg/day were smaller than those from control and 4 mg/kg/day groups, and their skeletons showed decreased ossification. Abnormal external development and visceral development were observed in living and dead fetuses exposed in utero to CSO at dose levels as low as 8 mg/kg/day. Based on these data, 4 mg/kg/day represents the No-Observed-Adverse-Effect-Level for both maternal and developmental toxicity.

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