The release of [3H]dopamine (DA) and [14C]acetylcholine (ACh) was monitored from single slices of the rabbit striatum. In all cases, the evoked overflow of ACh showed a higher peak and was of shorter duration than that of 3H products. For ACh, the release per pulse showed a marked decline with increasing frequency of stimulation, whereas flat frequency-release curves were obtained for DA. At 0.1 and 1 Hz the evoked overflows of ACh were 15 and 7 times greater, respectively, than those of DA. Haloperidol (0.03 microM) and sulpiride (1 microM) produced large increases in the evoked overflow of DA and ACh at 3 and 10 Hz; little effect was observed at lower frequencies. These results indicate that the frequency-release curves for DA and ACh are different and that at high frequencies the slope of the curves is modified by activation of pre- and postsynaptic DA receptors. Apomorphine inhibited in a concentration-dependent fashion the evoked overflow of DA and ACh; greater inhibition was obtained at lower frequencies of stimulation. At 0.3 Hz the DA agonist was two times more potent in inhibiting DA than ACh overflow (IC50: 12.0 +/- 2.2 versus 22.0 +/- 2.8 nM; p less than 0.01). The greater sensitivity of pre- than postsynaptic sites to apomorphine was also seen at higher frequencies (3 Hz). Benztropine (1 microM) reduced the evoked overflow of ACh at 10 Hz, and enhanced that of 3H products at all rates of stimulation (0.3-10 Hz). These results suggest that the release of DA and ACh is regulated by dopaminergic receptors. They also indicate that the effects of DA agonists and antagonists and of uptake inhibitors on DA and ACh release are highly dependent on the frequency of stimulation used.