The potential utility of adenoviruses for the treatment of chronic neurological disease is controversial due to reports of vector-associated toxicity, inflammation, and transient transgene expression. To focus upon the mechanism by which transgene expression is lost, we injected increasing doses [1 x 10(6) to 1 x 10(9) infectious units (iu)] of a first-generation adenovirus vector expressing beta-galactosidase into the brains of immune-competent adult rats. Transgene expression was evaluated simultaneously with acute neuronal and glial cell cytotoxicity, and acute and chronic inflammation using immunohistochemistry, at 3 and 30 days post-vector administration. Our results show a clear threshold effect of viral dose upon the amount of transgene expression persisting by 30 days after vector administration. Below 10(8) iu, transgene expression remained stable over the 30-day period. Following infection of more than 10(8) iu, the extent of transgene expression at 30 days was inversely correlated with increasing viral dose. The severity of acute inflammation increased proportionally with increasing vector dose from 10(6) to 10(9) infectious units. In contrast, acute vector-mediated cytotoxicity and chronic inflammation were observed only above the threshold level of vector dose. Above 10(8) iu both the extent of the acute toxicity and the severity of the chronic inflammation were inversely correlated with transgene expression at 30 days. Thus, our data suggest that both an acute loss of cells through direct vector-mediated toxicity and the elicitation of chronic inflammation (but not acute inflammation) may account for the decline in transduction persistence at high vector doses.