The first step in the inactivation endocannabinoids consists of their being rapidly (t1/2 ≤5 min) cleared away from their extracellular molecular targets. Being lipophilic in nature, endocannabinoids can diffuse through the plasma membrane if their amount in the extracellular milieu is higher than their intracellular concentration. However, in order to be rapid, this process needs to be driven by controllable and selective mechanisms, such as either a membrane transporter protein, or an intracellular enzymatic process capable of rapidly reducing the intracellular concentration of the two compounds, or both (Figure 3). Indeed, AEA and 2-AG are taken up by cells via selective, saturable, temperature-dependent and Na+-independent ‘facilitated transport’ mechanisms, known as the anandamide membrane transporter(s) (AMT) (Di Marzo et al., 1994; Beltramo et al., 1997; Bisogno et al., 1997a; Hillard et al., 1997; Maccarrone et al., 1998; 2000a, 2000b; see also Fowler & Jacobsson et al., 2002 for a review). If one or more proteins is (are) responsible for these mechanisms, however, it (they) has (have) not yet been isolated nor cloned. Overall, the collected data suggest that the same mechanism mediates the cellular uptake of all endocannabinoids (Fezza et al., 2002; Huang et al., 2002; Wilson & Nicoll, 2002), although none of the selective inhibitors of the putative endocannabinoid transporter developed so far (De Petrocellis et al., 2000; Lopez-Rodriguez et al., 2001; Ortar et al., 2003) has been tested yet on virodhamine, 2-AGE and NADA. However, it must be emphasized that until molecular evidence for the AMT is found, its existence will have to be doubted bona fide. Some authors have suggested that intracellular degradation of AEA by the microsomial enzyme mostly responsible for its hydrolysis, the ‘fatty acid amide hydrolase’ (FAAH, see below), can be sufficient alone to drive the facilitated diffusion of this compound from the extracellular milieu into the extracellular space (Glaser et al., 2003). Nevertheless, several observations still strongly, albeit indirectly, support the existence of the AMT (for a more detailed review, see Hillard & Jarrahian, 2003): (1) several cell types can be found that can rapidly take up AEA from the extracellular medium even though they do not express FAAH; (2) several compounds have been developed that are capable of inhibiting AEA cellular uptake without inhibiting AEA enzymatic hydrolysis via FAAH (De Petrocellis et al., 2000; Di Marzo et al., 2001b, 2001c; Lopez-Rodriguez et al., 2001; Ortar et al., 2003); FAAH inhibitors enhance, and anandamide uptake inhibitors inhibit, anandamide accumulation in cells (Kathuria et al., 2003); (3) substances that inhibit the AMT enhance the effect of AEA that are exerted at extracellular sites (i.e. at CB1 receptors) and inhibit those that are exerted at intracellular targets (i.e. VR1 receptors, see De Petrocellis et al., 2001) – if these compounds were simply acting by inhibiting FAAH, they should enhance AEA effects in both cases; (4) noladin and NADA, two endocannabinoids that are either resistant or refractory to enzymatic hydrolysis, respectively, are still taken up by cells in a temperature-dependent way, and their uptake is inhibited competitively by AEA (Fezza et al., 2002; Huang et al., 2002) and (5) lipopolysaccharide inhibits FAAH expression without affecting AEA cellular uptake (Maccarrone et al., 2001); conversely, nitric oxide, peroxynitrite and superoxide anions stimulate AEA cellular reuptake (Maccarrone et al., 2000a), while acute or chronic ethanol inhibit this process (Basavarajappa et al., 2003), without affecting FAAH activity. These data suggest that, although intracellular hydrolysis does greatly influence the rate of AEA-facilitated diffusion, the uptake process might still be mediated by a mechanism distinct from the one catalyzing AEA hydrolysis.