Minocycline, a semisynthetic derivative of tetracycline, displays beneficial activity in neuroprotective in models including, Parkinson disease, spinal cord injury, amyotrophic lateral sclerosis, Huntington disease and stroke. The mechanisms by which minocycline inhibits apoptosis remain poorly understood. In the present report we have investigated the effects of minocycline on mitochondria, due to their crucial role in apoptotic pathways. In mitochondria isolated suspensions, minocycline failed to block superoxide-induced swelling but was effective in blocking mitochondrial swelling induced by calcium. This latter effect might be mediated through dissipation of mitochondrial transmembrane potential and blockade of mitochondrial calcium uptake. Consistently, minocycline fails to protect SH-SY5Y cell cultures against reactive oxygen species-mediated cell death, including malonate and 6-hydroxydopamine treatments, but it is effective against staurosporine-induced cytotoxicity. The effects of this antibiotic on mitochondrial respiratory chain complex were also analyzed. Minocycline did not modify complex IV activity, and only at the higher concentration tested (100 microM) inhibited complex II/III activity. Other members of the minocycline antibiotic family like tetracycline failed to induce these mitochondrial effects.