Pathways of transduction employed by receptors for sphingosine 1-phosphate (S1P) are identified by the nature of second messengers and/or downstream targets regulated and, more formally, by direct assays of heterotrimeric G protein activation. The different methods generally agree. S1P₁ couples to members of the G_i family, apparently selectively, although reported pertussis toxin (PTX)-insensitive actions make categorical statements regarding exclusivity difficult. S1P₂ and S1P₃ couple to members of the G_i, G_q, and G_12/13 families. S1P₄ couples to G_i and possibly G_12/13, while S1P₅ couples to G_i and G_12/13 but not to G_q. In virtually all circumstances, coupling of S1P receptors to G_i is reflected in PTX-sensitive inhibition of adenylyl cyclase, activation of extracellular-regulated kinases (ERKs), and, depending on the cell, activation of phospholipase C (PLC). Coupling to G_q is reflected in PTX-insensitive activation of phospholipase C. Coupling to G_12/13 is reflected in activation of Rho and subsequent activation of serum response factor (SRF). Specific linkages have been verified in almost all instances by receptor-promoted [³⁵S]GTPγS/GDP exchange on identified G proteins.