1. We recently described that several 2-(2,5-dimethoxy-4-substituted phenyl)ethylamines (PEAs), including 4-I=2C-I, 4-Br=2C-B, and 4-CH(3)=2C-D analogs, are partial agonists at 5-HT(2C) receptors, and show low or even negligible intrinsic efficacy at 5-HT(2A) receptors. These results raised the proposal that these drugs may act as 5-HT(2) antagonists. 2. To test this hypothesis, Xenopus laevis oocytes were microinjected with the rat clones for 5-HT(2A) or 5-HT(2C) receptors. The above-mentioned PEAs and its 4-H analog (2C-H) blocked the 5-HT-induced currents at 5-HT(2A), but not at the 5-HT(2C) receptor, revealing 5-HT(2) receptor subtype selectivity. The 5-HT(2A) receptor antagonism required a 2-min preincubation to attain maximum inhibition. 3. All PEAs tested shifted the 5-HT concentration-response curves to the right and downward. Their potencies varied with the nature of the C(4) substituent; the relative rank order of their 5-HT(2A) receptor antagonist potency was 2C-I>2C-B>2C-D>2C-H. 4. The present results demonstrate that in X. laevis oocytes, a series of 2,5-dimethoxy-4-substituted PEAs blocked the 5-HT(2A) but not the 5-HT(2C) receptor-mediated responses. As an alternative hypothesis, we suggest that the psychostimulant activity of the PEAs may not be exclusively associated with partial or full 5-HT(2A) receptor agonism.