We studied the histologic and ultrastructural features of embryonic chick cartilage after the cartilage had been incubated in serum-free medium that contained hormones and growth factors known to stimulate in vitro cartilage growth. Pelvic cartilages from 9 d chick embryos were incubated in BGJb ( Fitton -Jackson modification) medium alone (control) or medium containing one of the following: N6 monobutyryl cyclic AMP 0.5 mM, forskolin 100 microM, triiodothyronine (T3) 10 nM, insulin 45 nM, or somatomedin C 0.67 nM. At the end of 3 d of incubation the cartilages were fixed in buffered formalin. Significant growth (increases in size, wet and dry weight) was seen with each treatment group. N6-Monobutyryl cAMP treated cartilage had an increased number of flattened immature chondrocytes with large nuclei and prominent nucleoli. The histologic and ultrastructural features of forskolin treated cartilage were indistinguishable from N6-monobutyryl cAMP treatment. The T3 treated cartilage contained large hypertrophic chondrocytes with prominent lacunar typical of mature cartilage. T3 treated cartilage had considerable vacuole formation and dilated endoplasmic reticulum. Insulin and somatomedin treated cartilage had histologic appearance similar to control cartilage. Thus, the effects of various hormones on embryonic cartilage growth in vitro can be separated as to whether growth is the result of chondrocytic hyperplasia (cyclic AMP mediated), chondrocytic hypertrophy with maturation (T3), or a combination of both hyperplasia and hypertrophy (insulin and somatomedin-C).