A cooperative regional study involving 850 litters was conducted at five experiment stations (Arizona, Florida, Oklahoma, Texas, and Virginia) to assess the effects of feeding 220 mg/kg of chlortetracycline (CTC) from 1 wk before to the initiation of the breeding season to 15 d after the breeding season and(or) from 110 d of gestation through lactation on reproductive performance of sows (2 x 2 factorial). Sows were fed 1.82 kg/d (2.27 kg during December, January, and February) during the breeding period and before farrowing. Feed was consumed on an ad libitum basis during lactation. Feeding CTC during the breeding season increased litter size at birth (10.8 vs 10.3; P < .05) and decreased feed consumption (5.4 vs 5.5 kg/d; P < .05) in the subsequent lactation period. Feeding CTC during lactation reduced lactation weight loss in sows (4.3 vs 6.1 kg; P < .07). Feeding the antibiotic at breeding had no effect on conception rate; however, feeding the antibiotic during lactation improved subsequent conception rate at the first service (80 vs 73%; P < .10) and overall conception rate (89 vs 84%; P < .05). This indicates that some carryover effect of feeding CTC during lactation occurred during the subsequent breeding period. Station x treatment interactions were observed for survival rate to 21 d (P < .05) and to weaning (P < .06). Overall, survival rates were not greatly affected by antibiotic feeding. No treatment interaction was observed for any trait measured, indicating that the effects of CTC during the breeding and lactation periods were independent.(ABSTRACT TRUNCATED AT 250 WORDS)