Epidemiological studies have shown that maternal infection can increase the risk for mental illness in the offspring. In a mouse model of maternal respiratory infection with influenza virus, the adult offspring display striking behavioral, pharmacological and histological abnormalities. Although influenza primarily infects the respiratory system, there are reports of viral mRNA and protein in the fetus of infected pregnant animals. To determine the extent of viral spread following maternal respiratory infection, we used RT-PCR to assay various maternal and fetal tissues for influenza A mRNAs coding for neuraminidase, non-structural protein 2, nuclear protein and matrix protein. While infected maternal lungs exhibit uniformly very strong signals, placentae are only rarely positive, and viral RNAs are not detectable in fetal brains from infected mothers. Thus, the effects of maternal infection on fetal brain development are likely to be indirect, probably involving the maternal inflammatory response.