At a time when pharmaceutical companies are having trouble finding new low MW drugs and when biologics are becoming more common, animal venoms could constitute an underexploited source of novel drug candidates. We looked for identifying novel animal toxins active against G protein-coupled receptors (GPCR), the most frequently exploited class of treatment targets, with the aim to develop novel research tools and drug candidates. Screening of green mamba ($Dendroaspis\ angusticeps$) venom against adrenoceptors identified two novel venom peptides. $\rho$-Da1a shown an affinity of 0.35 nM for the $\alpha$1a-AR while $\rho$-Da1b displayed affinities between 14 and 73 nM for the three $\alpha$2-ARs. These two venom peptides have sequences similar to those of muscarinic toxins and belong to the three-finger-fold protein family. $\alpha$1a-AR is the primary target for the treatment of prostate hypertrophy. In vitro and in vivo tests demonstrated that ρ-Da1a reduced prostatic muscle tone as efficiently as tamsulosin (an antagonist presently used), but with fewer cardiovascular side effects. $\alpha$2-ARs are the prototype of GPCRs not currently used as treatment targets due to a lack of specific ligands. Blockage of these receptors increases intestinal motility, which may be compromised by abdominal surgery and reduces orthosteric hypotension. In vitro and in vivo tests demonstrated that ρ-Da1b antagonizes $\alpha$2-ARs in smooth muscles and increased heart rate and blood catecholamine concentrations. These results highlight possible exploitation of $\rho$-Da1a and $\rho$-Da1b in important pathologies.