Connexins (Cx) form gap junction channels made up of two connexons (hemichannels) from adjacent cells. Although their biophysical characteristics and physiological importance remain debated, unopposed hemichannels may open towards the extracellular space upon stimulation by e.g. removal of extracellular divalent cations and allow transmembrane flux of fluorescent dyes and physiologically relevant molecules, such as ATP and ions. Cx43 and Cx30 are the major astrocytic connexins and both display hemichannel activity when heterologously expressed in Xenopus oocytes. While Cx43 hemichannels, in this setting, display a rather selective permeation profile, Cx30 hemichannels are permeable to both fluorescent dyes, atomic ions, ATP, glucose, and glutamate. Protein kinase C (PKC) (dys)regulation of Cx43 gap junctions is implicated in heart disease and ischemic stroke and could then, in addition, affect the hemichannel activity of the astrocytic connexins. To determine phosphorylation-dependent regulation of Cx30 and Cx43 hemichannels, these were heterologously expressed in Xenopus laevis oocytes and opened with divalent cation-free solution. Cx43 hemichannel activity was unaltered by both inhibition/activation of PKC and by mutational disruption of the proposed PKC-phosphorylation sites. Cx30 hemichannel activity, in contrast, was down-regulated by PKC activation, in a manner suggesting PKC-mediated channel closure. No single PKC consensus site could be assigned to this regulatory property by mutational analysis, suggesting that PKC-dependent regulation of Cx30 may rely on phosphorylation at multiple sites. In conclusion, Cx30, but not Cx43, hemichannels close upon PKC activation, illustrating that connexin hemichannels display not only isoform-specific permeability profiles but also isoform-specific regulation by PKC.