One of the objectives in our laboratory is to develop a system for regulated expression of recombinant genes encoding therapeutic secreted proteins from vectors injected into muscle. Adenoviral and adeno-associated viral vectors expressing erythropoietin or growth hormone from constitutive promoters were used to transduce skeletal muscle in mice with subsequent application to rhesus monkeys. Peak expression from adenoviral vectors is much higher than with AAV although it is less stable. In monkeys, serum erythropoietin from adenoviral vectors dropped 3–4 logs over the course of 1 year while that from AAV dropped only 4-fold over 11/2 years. For regulated expression of erythropoietin, mice and rhesus monkeys were administered two previously described rAAV vectors (Science 283, 88– 91; 1999). One vector constitutively expresses two chimeric proteins. These proteins are biologically inactive until they form a complex mediated by the presence of rapamycin. This ternary drug-protein complex functions as a transcriptional activator recognizing a unique DNA binding site present on the second vector upstream of the erythropoietin cDNA. Erythropoietin expression is controlled by dose and frequency of rapamycin administration. Six monkeys have received the described regulated expression system. In four monkeys, two cycles of rapamycin inducible expression have been observed with subsequent extinction of induction after 100 days following vector administration. One monkey has continued to show regulated expression of erythropoietin over 300 days with 9 cycles of intermittent drug administration while the other monkey has undergone 11 cycles of pharmacologic regulated erythropoietin expression for over 400 days. Our data support the feasibility of pharmacologically regulated transgene expression in primates. Mechanisms to explain heterogeneity in the stability of the regulated system in non-human primates is under evaluation. We are also developing modified vectors to improve the efficiency of the regulated system and screening analogs of rapamycin with superior safety profiles.