In ovine basilar arterial smooth muscle cells (SMCs), the fetal "big" Ca2+-activated K+ (BK) channel activity is significantly greater and has a lower Ca2+ setpoint than BK channels from adult cells. In the present study, we tested the hypothesis that these differences result from developmentally regulated phosphorylation of these channels. Using the patch-clamp technique and a novel in situ enzymological approach, we measured the rates and extents of changes in BK channel voltage activation from SMC inside-out patch preparations in response to selective activation and inhibition of channel-associated protein phosphatases and kinases (CAPAKs). We show that BK channel activity is modulated during development by differential phosphorylation and that the activities of CAPAKs change substantially during development. In particular, excised membrane patches from adult SMCs exhibited greater protein kinase A activity than those from a fetus. In contrast, fetal SMCs exhibited greater protein kinase G activity and phosphatase activity than adult SMCs. These findings extend our previous observation that the BK channel Ca2+ setpoint differs significantly in adult and fetal cerebrovascular myocytes and suggest a biochemical mechanism for this difference. In addition, these findings suggest that the functional stoichiometry of CAPAKs varies significantly during development and that such variation may be a hitherto unrecognized mechanism of ion channel regulation.