Folic acid (FA) and certain of its reduced congeners produce excitatory effects when applied to neuronal tissue. Recent evidence has suggested that folates have other biological properties in common with the excitatory amino acids. The purpose of this study was to determine the activity of folate compounds in a system sensitive to excitatory amino acids. Bilateral injection of folic acid into the nucleus accumbens resulted in a marked increase in locomotor activity at doses of 2.5 and 5 micrograms. Larger doses resulted in behavioral responses, such as body tremor and labored breathing, which interfered with the locomotor response. Similarly, 5-formyltetrahydrofolic acid (FTHF) produced a marked hypermotility response after bilateral injection into the nucleus accumbens (2.5-25 micrograms), while dihydrofolic acid, tetrahydrofolic acid, and 5-methyltetrahydrofolic acid were ineffective. Pretreatment with reserpine (10 mg/kg, i.p.) markedly reduced the hypermotility response elicited by folic acid and FTHF as did pretreatment with haloperidol in both peripheral (0.8 mg/kg) and direct (5 micrograms) injection into the nucleus accumbens. In addition, injection of muscimol (30 ng), which depresses hypermotility induced by dopamine and amphetamine, produced a significant decrease in the hypermotility response produced by folic acid. In contrast, pretreatment with phentolamine (5 mg/kg, i.p.) or propranolol (4 mg/kg, i.p.) did not decrease folic acid or FTHF-induced responses. These results suggest that folic acid and FTHF produce an increase in locomotor activity by facilitating dopaminergic neurotransmission in the nucleus accumbens, possibly by inducing the release of dopamine from the nerve terminals. Thus, these folates have effects similar to those of the excitatory amino acids when injected into the nucleus accumbens.