The D1 dopamine (DA) receptor (D1) is phosphorylated by protein kinase C (PKC), yet the role of PKC in D1 signaling is undefined. We investigated PKC effects on the D1 using transiently transfected cells. Activation of PKC by phorbol esters (PE) increased D1 phosphorylation. Co‐transfection of the D1 with PKC isoforms α, βI, γ, δ, ε, and λ resulted in greater D1 phosphorylation, whereas PKC isoforms μ, ν, η, and ζ were without effect. Activation of an M1 muscarinic receptor resulted in increased D1 phosphorylation, indicating heterologous phosphorylation by PKC. PKC effects on D1 signaling were evaluated using cAMP assays. Surprisingly, both activation and inhibition of PKC were found to potentiate DA‐stimulated cAMP accumulation. The potentiation is specific to D1 activation, since PE‐treatment had no effect on forskolin‐stimulated cAMP levels. To focus on D1‐specific PKC effects, we performed site‐directed mutagenesis of the D1 in order to identify the PKC phosphorylation sites. Five sites were identified: S259, S397, S398, S417, and S421. Simultaneous mutation of these sites eliminated PKC phosphorylation of the D1 resulting in increased G protein coupling and cAMP accumulation which was no longer potentiated by PKC inhibitors. These data suggest that constitutive PKC phosphorylation of the D1 regulates the signaling tone of the receptor.