Changes in myosin light chain kinase (MLCK) and calmodulin (CaM) mRNAs have been evaluated during estrogen-mediated differentiation of the chicken oviduct. Also examined were acute changes that occur in oviduct RNA from animals stimulated with estrogen, withdrawn from hormone and then injected for 1, 2, and 4 days with synthetic estrogen [diethylstilbestrol (DES)], progesterone (P), or testosterone (T). Small changes were noted in both CaM and MLCK RNAs during primary stimulation when oviduct cells are actively dividing. On the other hand no significant changes were observed during secondary stimulation regardless of the steroid hormone injected. These data support the contention that CaM and MLCK are constitutively expressed but vary as a function of cell cycle. The MLCK mRNA is 5.5 kilobases (kb) but the MLCK cDNA also hybridizes to an oviduct RNA 2.7 kb long. This RNA species is acutely regulated by estrogen, P, and T but in a manner different from that of ovalbumin mRNA. The magnitude of stimulation of the 2.7 kb mRNA by diethylstilbestrol and T is greater than that of ovalbumin whereas changes in response to P are similar. The 12- to 16-fold increase of the 2.7 kb mRNA in response to T is the largest effect reported for this hormone acting on oviduct. The 2.7 kb mRNA encodes an unknown protein yet contains a 520 nucleotide segment that is highly homologous with the COOH-terminal coding portion of the MLCK mRNA. Since this homology does not include either catalytic or CaM-binding domains of MLCK, it is unlikely that the 2.7 kb mRNA encodes a CaM-dependent protein kinase.(ABSTRACT TRUNCATED AT 250 WORDS)