The present study describes the effects of immobilization stress on the induction of supernumerary ribs. To isolate the sensitive period of SNR induction, Sprague-Dawley rats and CD-1 mice were treated with 300 mg/kg and 1500 mg/kg, respectively, of sodium salicylate on single days 7-11 of gestation. In the rat, day 10 was found to be the sensitive period of lumbar rib induction while 9 was critical in the mouse. In a second set of experiments, maternal stress was accomplished by restraining two groups of gravid females in the supine position for 12 hours on the predetermined sensitive day. One group was immobilized from 9am-9pm, while the second group was restrained from 9pm-9am. An increase in supernumerary ribs was noted in stressed mice but not in rats. The 9am-9pm mouse group exhibited the highest increase in supernumerary ribs (41%) as well as significant incidences of fused ribs and exencephaly. Results suggest that supernumerary ribs may be the indirect result of agent-induced, generalized maternal stress in the CD-1 mouse. (Copyright (c) 1986 Alan R. Liss, Inc.)