Neurobehavioral assessment of 2,4-dichlorophenoxyacetic acid (2,4-D) in rats.

2,4-Dichlorophenoxyacetic acid (2,4-D), an organic acid commonly used as a herbicide, was assessed for behavioral and neurological toxicity in male Fischer-344 rats. When given by gavage twice weekly for 5 weeks, 2,4-D produced a significant increase in fore- and hindlimb grip strength. Other neurological measurements such as negative geotaxis, hindlimb splay, motor activity, and startle responsiveness, were not affected. A second experiment, in which rats were dosed 5 days per week for 4 weeks, replicated the finding that 2,4-D increases grip strength of rats. In addition, the neurological effect was found to dissipate with time following cessation of dosing. That 2,4-D increases grip strength may be related to the observation that it induces myotonia, a condition characterized by difficulty in relaxation of skeletal musculature following forceful contraction.