The general amino acid transport system of Saccharomyces cerevisiae functions in the uptake of neutral, basic, and acidic amino acids. The amino acid analogue N-5-chloroacetyl-L-ornithine (NCAO) has been tested as a potential site specific reagent for this system. l-Tryptophan, which is transported exclusively by the general transport system, was used as a substrate. In the presence of glucose as an energy source, NCAO inhibited tryptophan transport competitively (Ki=80 μM) during short time intervals (1-2 min), but adding 100 μM NCAO to a yeast cell suspension resulted in a time-dependent activation of tryptophan transport during the first 15 min of treatment. Following the activation a time-dependent decay of tryptophan transport activity occurred. Approximately 80% inactivation of the system was observed after 90 min. When a yeast cell suspension was treated with NCAO in the absence of an energy source, an 80% inactivation of tryptophan transport occurred in 90 min. The inactivation was noncompetitive (Ki ⋍ μM) and could not be reversed by the removal of the NCAO. Addition of a fivefold excess of L-lysine during NCAO treatment or prevented inactivation of tryptophan transport. Under parallel conditions of incubation, other closely related transport systems were not inhibited by NCAO.