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The activity of aminoacid transport, as measured by alpha-aminoisobutyrate uptake, has been studied in confluent myocardial cell cultures exposed to different oxygen tensions. The results obtained indicate that the rate of cellular uptake and accumulation of the inert aminoacid increase with time as the fraction of oxygen is reduced. When alpha-aminoisobutyrate was added in presence of all other aminoacids of the medium, the effect of oxygen was also evident, suggesting a mechanism which overcomes the competitive action of the other aminoacids assigned to the same transport system of alpha-aminoisobutyrate (A system). The modulation of aminoacid transport activity may represent one of the possible mechanisms by which environmental oxygen affect the rate of cellular protein synthesis.