Many sarcomeric proteins in the myocardium alter their isoform pattern during perinatal development to adjust to the intensified pump function of the postnatal heart. These changes also involve the giant protein titin/connectin. Here we show by low-percentage polyacrylamide-gel electrophoresis that developmentally regulated switching of cardiac titin/connectin size occurs in the hearts of mouse, rat, pig, and chicken. Mammalian hearts express, well before birth, large foetal (approximately 3.7 MDa) N2BA-titin/connectin isoform but no N2B-isoform (3.0 MDa). During perinatal heart development the 3.7-MDa N2BA-isoform is replaced by a mix of smaller isoforms. At birth a plethora of intermediate-size N2BA-isoforms appears together with the N2B-isoform. In postnatal heart development the larger-size N2BA-isoforms disappear and smaller-size N2BA-isoforms are upregulated, whereas the proportion of N2B-titin/connectin increases to species-specific adult levels. The time courses of isoform switching are faster in small than in large mammals. Titin/connectin isoform switching also takes place in developing chicken hearts, but the largest embryonic isoform found here was less than 3.4 MDa. At hatching, various smaller-size isoforms appeared and within a week the adult expression pattern was established representing a major 3.0-MDa isoform and a minor 3.15-MDa isoform. The ratio between the two adult isoforms differed between the left ventricle and the right atrium. The perinatal changes toward smaller cardiac titin/connectin isoforms in mammals and chicken greatly increase the myofibrillar passive tension of postnatal hearts. Plasticity of titin/connectin at approximately the time of birth thus affects myocardial mechanics but could also be an important factor in developmentally regulated assembly and signalling processes.