Injury to the cerebral cortex of the rat brain has been shown to induce the expression of neurotrophic factors for dissociated peripheral and central neurons in culture. We confirm this phenomenon and report that Xenopus laevis oocytes injected with mRNA extracted from wounded rat cortex expressed similar neurotrophic activity. To detect the low amounts of neurotrophic factors that could be expected from the oocyte translation system, a miniaturization of the assay for neurotrophic and cell-surviving activity was developed using Terasaki microtiter plates for culture of chicken embryo sympathetic ganglion cells. Messenger RNA (mRNA) was size-fractionated on a sucrose gradient and RNAs from each fraction were injected into oocytes. Neurotrophic activity was recovered from the homogenates and from the incubation media of oocytes injected with mRNA from 7 day post-lesion cortex. Messenger RNAs in the active fractions ranged in size from 0.8 to 1.8 kb. As much as 20% of the activity was secreted by the oocytes. No significant neurotrophic activity was detected from oocytes injected with mRNA fractions extracted from the cortex of control rats or from other gradient fractions from post-lesion cortex.