We observed that rat thymocyte cultures exposed to 1.0 - 2.5 microM tri-n-butyltin methoxide (TBT) exhibited a rapid time- and concentration-dependent induction of apoptosis, with > 85% of cells exhibiting reduced DNA content within 1 hr after exposure to 2.0 - 2.5 microM TBT. Moreover, with continuous exposure to TBT, the DNA content of apoptotic nuclei increased with time, suggesting a reduced ability of DNA fragments to leave the nucleus of TBT-exposed cells following detergent-mediated cytolysis, possibly as a consequence of membrane/cytoplasm fixation. In contrast, exposure to 1.0 microM dexamethasone phosphate (DEX) resulted in a gradual time-dependent increase to approximately 45% induction of apoptosis by 6 hr (versus approximately 15% spontaneous induction in controls). However, simultaneous exposure to TBT and DEX resulted in a decreased response: TBT concentrations between 0.1 and 0.5 microM (which alone did not induce apoptosis) reduced the ability of DEX to induce apoptosis; at TBT concentrations greater than or equal to 1.0 microM, simultaneous exposure to DEX substantially decreased the extent of both TBT-induced cytotoxicity and apoptosis. Furthermore, while treatment with cycloheximide (CHX), a protein synthesis inhibitor, or H-7, a protein kinase C (PKC) inhibitor, completely blocked DEX-induced apoptosis, neither significantly reduced induction of apoptosis by TBT.