Crude and refined petroleum contain a complex mixture of aliphatic, aromatic, polyaromatic and heterocyclic hydrocarbon compounds. The objective of our research was to investigate early-stage biochemical changes in rats exposed to low dosages of petroleum hydrocarbons. The animals were repeatedly exposed, per oral by gavage, to low dosages (0.5-2.5 ml/kg) of an Alberta crude oil (ACO) and their general health and systemic biochemical parameters were assessed. Rats exposed to these doses of ACO did not show any apparent symptoms of intoxication. Similarly, no significant changes were observed in clinical parameters of systemic impairment. Systemic biochemical assessment has shown that ACO exposure caused marked changes in the activities of several cytochrome P-450 (CYP)-linked polysubstrate monooxygenase enzymes in liver, kidney and lung tissues. Exposure to ACO caused dose-dependent increases in the hepatic activities of ethoxyresorufin-O-deethylase, a CYP 1A1/A2-linked enzyme; pentoxyresorufin-O-dealkylase, a CYP 2B-linked enzyme, and ethoxycoumarin-O-deethylase, a CYP 2B/1A-linked enzyme. Temporal assessment showed that these systemic biochemical changes were reversible in nature. Analysis of biomarker chemicals provided evidence that in exposed animals petroleum hydrocarbons were mainly distributed in the adipose tissues.