Abstract Alizarin red-S stained skeletons of the fetuses of the streptozotocin induced (on days 2, 6 and 8 of gestation) diabetic and insulin and buffer treated control rats were examined on day 20 of gestation for malformations and hypoplasia. Hypoplasia of both the neurocranial and viscerocranial bones was significant in all 3 diabetic groups. Insulin treatment had a remarkable rescue effect on fetuses of the day 8 group; a moderately similar response was found in others. Inhibition of ossification of the scapula, humerus, radius and ulna was relatively less remarkable when compared to that of the homologous bones of the hind-limbs. However the metacarpals were affected more than the metatarsals. Day 6 of gestation was particularly susceptible for retardation of sternal ossification. Agenesis, hemi- and double hemivertebrae were also observed in a few cases. Reduction in caudal sacral and coccygeal vertebral development was remarkable. Incidence of the 14th rib was highest in the day 6 group and insulin administration significantly reduced this abnormality. Meticulous metabolic control had beneficial effect over the growth retarding effects of maternal diabetes but did not completely prevent the malformations.