One of the toxic symptoms of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) is reduction in metabolic rate and subsequent growth retardation. Acetylcholine (ACh) serves as an essential growth factor to facilitate amino acid transport and to promote fetal growth. Hydatidiform mole lacks the capacity for synthesis of ACh, and inhibition of ACh synthesis depresses placental amino acid transport. Therefore, we studied the formation of 2,4,5-acetylcoenzyme A (2,4,5-T-CoA) by acetylcoenzyme A synthase (ACoAS) and the formation of 2,4,5-T-ACh by human placental choline acteyltransferase (ChA) from 2,4,5-T-CoA and choline. In these studies, the widely used analog of 2,4,5-T as an herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), was also included. These studies have the following results (M +/- S.D.; N,6):1) The enzymatic rates of formation of acetyl-CoA, 2,4,5-T-CoA, and 2,4-D-CoA by ACoAS were 32 +/- 4, 23 +/- 3 and 26 +/- 8 nmol/mg protein/5 min., respectively; 2) There were no significant differences in the maximal amounts (nmol/mg protein) of acetyl-CoA (128 +/- 4), 2,4,5-T-CoA (125 +/- 8) and 2,4-D-CoA (96 +/- 6) formed during the reaction period of 50 min.; 3) 14C-2,4-ACh was formed from 14C-2,4-D-CoA and choline by placental-ChA; 4) Low concentrations (EC50 1-2 microM) of synthetic 2,4,5-T-ACh and 2,4-D-ACh decreased the contraction heights of the rat phrenic nerve-hemidiaphragm when the nerve or the muscle was electrically stimulated, and 5) Similar results were obtained with 2,4,6-T-ACh, an analog of 2,4,5-T-ACh. These observations indicate that chlorophenoxyherbicides form false cholinergic messengers in the nerve, muscle and placenta. These false cholinergic messengers can be formed at both muscarinic and nicotinic synaptic sites and also in non-neuronal cells, where ACh plays an important regulatory role as a local hormone, and act as blocking agents. These results will partially explain myotonia, ventricular fibrillation and fetal growth retardation induced by these herbicides.