Prosopis juliflora is used for feeding cattle and humans. Intoxication with the plant has been reported, and is characterized by neuromuscular alterations and gliosis. Total alkaloidal extract (TAE) was obtained using acid/basic-modified extraction and was fractionated. TAE and seven alkaloidal fractions, at concentrations ranging 0.03-30 microg/ml, were tested for 24h on astrocyte primary cultures derived from the cortex of newborn Wistar rats. The MTT test and the measure of LDH activity on the culture medium, revealed that TAE and fractions F29/30, F31/33, F32 and F34/35 were cytotoxic to astrocytes. The EC(50) values for the most toxic compounds, TAE, F31/33 and F32 were 2.87 2.82 and 3.01 microg/ml, respectively. Morphological changes and glial cells activation were investigated through Rosenfeld's staining, by immunocytochemistry for the protein OX-42, specific of activated microglia, by immunocytochemistry and western immunoblot for GFAP, the marker of reactive and mature astrocytes, and by the production of nitric oxide (NO). We observed that astrocytes exposed to 3 microg/ml TAE, F29/30 or F31/33 developed compact cell body with many processes overexpressing GFAP. Treatment with 30 microg/ml TAE and fractions, induced cytotoxicity characterized by a strong cell body contraction, very thin and long processes and condensed chromatin. We also observed that when compared with the control (+/-1.34%), the proportion of OX-42 positive cells was increased in cultures treated with 30 microg/ml TAE or F29/30, F31/33, F32 and F34/35, with values raging from 7.27% to 28.74%. Moreover, incubation with 3 microg/ml F32, 30 microg/ml TAE, F29/30, F31/33 or F34/35 induced accumulation of nitrite in culture medium indicating induction of NO production. Taken together these results show that TAE and fractionated alkaloids from P. juliflora act directly on glial cells, inducing activation and/or cytotoxicity, stimulating NO production, and may have an impact on neuronal damages observed on intoxicated animals.