Dense gas in nearby galaxies. VII. The active nucleus of NGC 4945.

^12^CO, c^18^O, CN, HCN, N_2_H^+^, and HNCO transitions, tentative detections of C_3_H_2_, HC_3_N, and C^17^O transitions, and upper limits for transitions of other molecular species are reported from the nuclear region of the starburst galaxy NGC 4945. The bulk of the CO emission arises from gas with densities >1000 cm^-3^. A first map in a molecular high density tracer (HCN) indicates that low and high density molecular gas coexists in the central few 100 pc; area filling factors derived from CO and HCN are ~0.3 and ~0.01, respectively. The presence of two main molecular components is confirmed: One is associated with gas presumably forming an edge-on nuclear ring. The other component arises from a more quiescent environment. CN, HNCO, and H_2_O data indicate different physical conditions in the two outer lobes of the rotating ring. The redshifted lobe at ~700 kms/s appears to be higher excited, giving rise to enhanced CN N=2-1 and H_2_O emission. CO and HCN data also allow estimates of isotope ratios. The ^12^C/^13^C ratio is likely ~50, suggesting recent infall of gas toward the nuclear region. The ^16^O/^18^O and ^18^O/^17^O ratios are ~150 and >=8. This is explained by a high ^18^O abundance due to recent nucleosynthesis in massive stars; the starburst must have reached an advanced stage of evolution.