In order to evaluate disturbances of the respiratory control in the first year of life in children with a statistically enhanced risk of SIDS, substance P-like immunoreactivity (SPLI) in plasma and mean apnoea duration (MA) were examined. 4 groups of infants were investigated: Controls, full-term infants with anamnestic SIDS-risk factors, preterm infants with additional risk factors and preterm infants without such factors. Infants aged from -4(corrected age) to 63 weeks. SPLI in plasma was determined by a specific, homologous radioimmunoassay. The SPLI-level was significantly higher in controls (n = 41; means +/- SE = 36.37 +/- 4.86 pg/ml) than in preterm infants without (n = 21; 25.41 +/- 5.54 pg/ml) or with additional anamnestic risk factors (n = 111; 25.89 +/- 3.09 pg/ml). SPLI was higher in full-term SIDS-risk infants (n = 150; 30.73 +/- 2.35 pg/ml) than in the preterm groups. There is a significant age dependence in the groups full-term SIDS-risk infants and preterm infants with additional risk factors. During maturation the SPLI-level in plasma rises in these groups from lower values. The MA-values were determined by means of a daytime polygraphy. There is an age dependence of the MA-values during active sleep in full-term SIDS-risk infants and in preterm infants with additional anamnestic risk factors. In the age group 4-17 weeks (peak of SIDS frequency) in active sleep the MA-values were significantly higher in all 3 risk groups than in the controls.(ABSTRACT TRUNCATED AT 250 WORDS)