The appearance of neonate rat endocranial osteoblasts exposed in situ and fixed immediately was compared with that of similar osteoblasts organ cultured for short periods of up to 48 hours in control medium and serum alone, or with added parathyroid extract (PTE). Normal osteoblasts showed a range of variation in size and shape, degree of elongation and orientation. Culturing the osteoblasts resulted in an overall loss of elongation and ordering of the cells, and the production of dorsal ruffles which were more complex and larger in the longer culture times. PTE added to the culture medium caused an increase in cell elongation and a striking recordering of the osteoblasts into domains of parallel cells. The swirling patterns made by these domains were similar to those of the underlying bone collagen. Ruffles, where present, were small and more often peripheral than dorsal. The results indicate that fully differentiated osteoblasts are able to adapt very rapidly to survival and function in culture conditions, and that the endocytosis necessary for synthetic activity is suppressed by PTE although some cell movement may continue.