Pups from the strains 129SvEv/Bkl and C57BL/6/Bkl and F1 hybrids of these were subjected to an analysis of the development of swimming pattern (from the day of birth until 21 days old) to study the neuromuscular development of neonatal and juvenile mice of common laboratory strains in gene modification programmes. Swimming as a parameter of scrutiny was chosen because it requires total coordination of the body's muscles, and we found that the gradual change from a neonatal to an adult swimming pattern can be scored objectively. Five different parameters were scored: the position of the head in the water, the use of front legs, the use of hind legs, the use of the tail and whether or not the animals could maintain a straight course in the water or not. Each parameter could be objectively scored as 0 (neonatal), 1 (juvenile) or 2 (adult) level of development. Both the F1 hybrids C57BL/6/Bkl♂x 129SvEv/Bkl♀ and 129SvEv/Bkl♂x C57BL/6/Bkl♀ were fully developed swimmers after only 13 to 14 days, respectively, while it took until 19 or 20 days for the mice of the pure strains to develop an adult swimming pattern. The study also revealed some major differences in swimming pattern between the two strains. The 129SvEv/Bkl maintained a horizontal position in the water and were strong swimmers, whereas the C57BL/6/Bkl mice consistently exhibited a startle reaction when first introduced to the water, maintained a more vertical position in the water and tired easily.