A hormonal model of erythropoietin (Epo) delivery by use of an adenovirus vector was investigated. We constructed a replication-defective adenovirus carrying the monkey (cynomolgus) Epo cDNA under control of the Rous sarcoma virus long terminal repeat promoter. Fifty 8-week-old mice were injected with escalating doses of the recombinant virus from 10(6) to 10(10) plaque-forming units (pfu). Different modes of administration were studied. Intravenous (i.v.) injection was the most effective mode of administration and exhibited a dose-dependent response. After a single i.v. injection with high doses (5 x 10(9) and 10(10) pfu), a dramatic increase in hematocrit (Hct) and long-term Epo expression (6 months at this time) were observed. Intravenous administration with lower doses and intramuscular (i.m.) administration were inefficient or had a very transient effect. A localized muscle attrition prior to i.m. administration of 10(10) pfu enhanced Hct response. This initial study opens the way for high level and durable Epo therapy by gene transfer. Moreover, this recombinant virus provides a convenient means to study the efficacy, duration, and safety aspects of hormonal delivery by an adenoviral vector.