The survival requirements of motoneurones obtained from differently aged avian embryos was analysed, in both heterogeneous cultures of motoneurones with spinal cord cells and homogeneous cultures of motoneurones obtained by cell sorting. It was found that medium conditioned by contact with immature astrocytes could maintain more than 75% of the motoneurones plated from 5-day embryos for two days; however, this astrocyte medium could not maintain motoneurones plated from 8-day embryos above control levels at two days. In contrast, medium conditioned by contact with myotubes could not maintain motoneurones plated from 5-day embryos above control levels for two days; this myotube medium could maintain more than 70% of the motoneurones plated from 8-day embryos for two days. The change in the receptivity of motoneurones to astrocyte-conditioned medium may be due to their ageing. Thus, motoneurones from 6-day embryos could not be sustained above control numbers in culture for 4 days with astrocyte media, in the same way as motoneurones from 8-day embryos degenerate by two days. In contrast, more than 70% of motoneurones plated from 6-day embryos could be maintained in culture for 4 days with myotube media in the same way as motoneurones from 8-day embryos for two days. The results indicate that motoneurones from 5-day embryos are dependent for their survival on immature astrocytes but that this switches to a dependence on myotubes during the normal motoneurone death period from 6 days to 10 days of embryonic age.