A ribosomal system for [3H]leucine incorporation was isolated from the rat oviduct in order to study the possible effect of GABA on [3H]leucine incorporation during the estrous cycle. The system showed an absolute requirement for Mg2+ and about 50% dependence on an energy source. Optimal [3H]leucine incorporation occurred under 3-6 mM Mg2+ and 100 mM K+ and was higher in diestrous-1 than in estrous or proestrous. GABA (10 mM) had no effect on [3H]leucine incorporation in any of the three estrous phases studied.