During the last three decades, the necessity to measure the release of neurotransmitters in vivo in the central nervous system (CNS) has prompted the development of innovative techniques for sampling the extracellular fluid in the brain of experimental animals. Historically, one of the methods that evolved for this purpose was push-pull perfusion, which involved the stereotaxic insertion of a push-pull cannula into a selected area of the brain. Originally described by Gaddum (1961), such cannulae consisted of two stainless steel tubmgs assembled in a concentric manner, i.e., a smaller push cannula inserted into an outer pull cannula. The continuous perfusion of a physiological fluid through the system allows the collection of consecutive perfusate samples from the pull tubing. For over 20 yr, the push-pull technique was used routinely in a variety of studies dealing with the in vivo activity of neurotransmitters and other endogenous factors in the brain (reviewed by Philippu, 1984, Gardner et al., 1993; Myers et al., 1997).