Adult vascular smooth muscle expresses 204-kD (SM1) and 200-kD (SM2) myosin heavy chain (MHC) isoforms. Fetal vascular smooth muscle expresses another 200-kD isoform, MHC-B, that appears to be developmentally regulated. The ontogeny of expression of these MHC isoforms in vascular and nonvascular smooth muscles is not fully understood and may differ. In the present report we examined the ontogeny of these isoforms in aortic and bladder smooth muscle from male fetal (n = 12, 119-140-d gestation; term 145 +/- 5 d) and neonatal (n = 12, 1-33 d) sheep. Tissues were analyzed for total and soluble protein contents. Actin, MHC, and MHC isoforms were analyzed by SDS-PAGE using 3-20% and 4% polyacrylamide gels, respectively. The expression of the adult and fetal 200-kD MHC isoforms were determined by Western analysis. Between 119 d gestation and 33 d neonatal, age-dependent increases (p < 0.02) occurred in bladder actin (16 +/- 0.8 versus 22 +/- 1.4 micrograms/mg of wet weight), MHC (6.5 +/- 0.2 versus 9.7 +/- 1.1) and both soluble (71 +/- 2.9 versus 92 +/- 6.3) and total protein (78 +/- 3.9 versus 103 +/- 5.5). Aortic smooth muscle actin (8.5 +/- 0.7 versus 17 +/- 1.1), MHC (3.1 +/- 0.4 versus 5.2 +/- 0.5), and soluble (44 +/- 2.3 versus 61 +/- 3.0) and total protein (87 +/- 5.8 versus 108 +/- 3.2) also increased (p < 0.01). Aortic SM1 increased (r = 0.79, p < 0.001) during this time, whereas expression of the 200-kD MHC fell (r = -0.79, p < 0.001). In contrast, bladder SM1 fell (r = -0.88, p < 0.001) as the 200-kD MHC rose (r = 0.88, p < 0.001). The type of 200-kD MHC isoform expressed also differed between tissue types; bladder expressed SM2 and little or no MHC-B throughout this phase of development, whereas fetal aorta appeared to express primarily MHC-B, which decreased as adult SM2 expression rose after birth. Expression of smooth muscle proteins and MHC isoforms are developmentally regulated and tissue-dependent, the latter perhaps reflecting developmental differences in organ growth and/or function.