In this paper we review some of our recent work on the structural and biochemical characterization of isoforms of the heavy chain of vertebrate smooth muscle myosin II. There exist both amino-terminal and carboxyl-terminal alternatively spliced isoforms of the smooth muscle myosin heavy chain (MHC). mRNA splicing at the 3' end generates two MHCs, which differ in length and amino acid sequence in the carboxyl terminus. We will refer to the longer, 204-kDa isoform as MHC204 and the shorter, 200-kDa isoform as MHC200. We found that MHC204, but not MHC200, can be phosphorylated by casein kinase II on a serine near the carboxyl terminus, suggesting that these isoforms may be differentially regulated. The physiological significance of this phosphorylation is not known. However, as demonstrated in this paper, phosphorylation does not appear to affect filament formation, velocity of movement of actin filaments by myosin in an in vitro motility assay, actin-activated Mg2+ ATPase activity, or myosin conformation. Our results also show that MHC204 and MHC200 form homodimers, but not heterodimers. Purified MHC204 and MHC200 homodimers are not enzymatically different, at least as measured using an in vitro motility assay. The amino-terminal spliced MHC204 and MHC200 isoforms are the result of the specific insertion or deletion of seven amino acids near the ATP-binding region in the myosin head. We refer to these isoforms as inserted (MHC204-I; MHC200-I) or noninserted (MHC204; MHC200), respectively. In contrast to the carboxyl-terminal spliced isoforms, the amino-terminal spliced inserted and noninserted myosin heavy chain isoforms are enzymatically different. The inserted isoform, which is expressed in intestinal, phasic-type smooth muscle, has a higher actin-activated Mg ATPase activity and moves actin filaments at a greater velocity in an in vitro motility assay than the noninserted MHC isoform, which is expressed in tonic-type vascular smooth muscle. The results presented in this review suggest that the alternative splicing of smooth muscle mRNA results in at least four different isoforms of the myosin heavy chain molecule. The potential relevance of these molecular isoforms to smooth muscle function is discussed.