['RhoA and Rho kinase are important modulators of microvascular tone.', 'We tested whether sphingosine kinase (Sphk1) that generates the endogenous sphingolipid mediator sphingosine-1-phosphate (S1P) is part of a signaling cascade to activate the RhoA/Rho kinase pathway. Using a new transfection model, we report that resting tone and myogenic responses of isolated resistance arteries increased with forced expression of Sphk1 in smooth muscle cells of these arteries. Overexpression of a dominant negative Sphk1 mutant or coexpression of dominant negative mutants of RhoA or Rho kinase together with Sphk1 completely inhibited development of tone and myogenic responses.'] Does sphingosine kinase modulate microvascular tone and myogenic responses through activation of RhoA/Rho kinase?