The resting and K+-stimulated release rates of endogenous dopamine (DA) have been measured in vivo at four different sites in the rat caudate putamen and nucleus accumbens. A push-pull cannula was inserted into the brain sites chosen, and the tissue was perfused with artificial cerebrospinal fluid (CSF) containing 2.6 or 40 mM KC1. The DA content of the perfusates was determined by a radioenzymatic procedure. DA release was significantly increased above unstimulated levels by 40 mM KC1 in all areas tested. Neither unstimulated nor K+-stimulated release rates varied significantly among the regions examined. K+-stimulated DA release was not significantly diminished by perfusing the tissue with calcium-free medium, suggesting that release was probably supported by residual amounts of calcium in the tissue.