Primary cultures of bovine adrenomedullary chromaffin cells were treated with the phosphorous acid ester, triphenyl phosphite (TPP), and the morphological changes assessed by transmission electron and scanning microscopy. Parallel studies were carried out with the cholinergic compound 0,0-diethyl-4 nitrophenyl phosphate (paraoxon) and the delayed neurotoxicant 0,0-diisopropylphosphorofluoridate (DFP). Transmission and scanning electron microscopy revealed that treatment with both of these organophosphorous compounds did not produce the ultrastructural effects that were seen with TPP. Treatment with 100 micromoles TPP for 4 or 24 hours caused a marked inhibition (90% relative to controls) of adenosine incorporation. Neither 100 micromoles paraoxon or 100 micromoles DFP had an inhibitory effect on incorporation. The results support a specific affect of the triphenylphosphite, TPP, not a general toxic effect of organophosphorous compounds since the cholinergic agent paraoxon and the delayed neurotoxic compound DFP did not alter the cells ultrastructure.