Modulation of gene expression via nucleic acid sequence-specific intervention represents a new paradigm for drug discovery and development. Ribozymes are small RNA structures capable of cleaving RNA target molecules in a catalytic fashion. A 2'-O-allyl-modified hammerhead ribozyme designed to cleave the messenger RNA of cytochrome P-450 3A2 was administered to rats via 0.25 mg intravenous injections to investigate the disposition of this compound. The chemically modified ribozyme binds to serum albumin and can be displaced by phosphorothioate oligonucleotides. A biphasic plasma clearance with a distribution half-life of 12 min and an elimination half-life of 6.5 h was observed. A volume of distribution of 2.1 l/kg indicates perfusion into tissues well beyond the vascular system. The chemically modified ribozyme can be detected intact in the plasma up to 48 h after injection. Metabolic degradation of the chemically modified ribozyme occurs at unmodified ribonucleotides, leaving the 2'-O-allyl-modified sites intact. Recovery of intact chemically modified ribozyme was 1.9% of the administered dose at 12 h along with significant metabolites. The renal clearance of the intact ribozyme is an average 34.3 ml/h. The tissue distribution of the chemically modified ribozyme at 48 h is primarily to kidney and liver but the only detected material is a single 27-mer metabolite that has been cut in the unmodified GAAA region. The brain concentration of the prominent 27-mer metabolite is greater than that observed in the lung or spleen. Examination of tissues reveals no morphological evidence of toxicity. These data strongly support the potential utility of synthetic, 2'-O-allyl-modified hammerhead ribozymes as therapeutic agents in vivo.