Simultaneously recorded pairs (from a single electrode) of cells in the medial septum (MS) vertical limb of the diagonal band of Broca (vDBB) were studied during the simultaneous occurrence of field activity in the hippocampal formation of urethane-anesthetized rats and following the systemic administration of atropine sulphate (ATSO4). The 2 types of field activities recorded were cholinergically mediated type 2 theta (theta) and large-amplitude irregular activity (LIA). The study had 3 objectives: (1) to determine if cells in close proximity in the MS/vDBB had a high probability of having similar discharge properties; (2) to determine the possible functional relationships occurring between pairs of MS/vDBB cells using standard cross correlational analyses; and (3) to determine the effects of ATSO4 on both the discharge properties of MS/vDBB cells and the interactions between these cells. Of the 143 cells recorded, 133 (93%) were theta-related and classified as theta-on or theta-off cells. theta-on cells had a significant increase in mean discharge rate during theta compared to their mean discharge rate during LIA and/or had a linear increase in discharge rate in relation to increases in theta frequency. theta-off cells had a significant increase in mean discharge rate during LIA compared to their mean discharge rate during theta and/or had a linear decrease in discharge rate in relation to increases in theta frequency. The remaining 10 (7%) cells were classified as nonrelated to theta. A probability analysis carried out on each of the separate criteria for classification revealed that several of these could predict whether or not cells were in close proximity of one another. Cells occurred in close proximity according to whether they had higher discharge rates during theta or LIA and according to whether or not they varied their discharge rates as theta frequency varied (linear-nonlinear). Cell discharge patterns of rhythmicity (phasic), or nonrhythmicity (tonic) on the other hand, were not good predictors of whether cells were in close proximity since there was an equal probability of a phasic cell being paired with either a phasic or a tonic cell. The presence of a phasic discharge pattern was, however, the main determinant of whether a correlation occurred between cell pairs. Cross correlations of the cell pair spike trains revealed that 50% of the 36 phasic-phasic cell pairs were correlated during both theta and LIA field activities. The characteristics of the cross correlation functions of these cell pairs (strong symmetrical correlations with sharp primary peaks or troughs occurring around the origin) suggested that they received shared excitatory and inhibitory inputs. The remaining 50% of the phasic-phasic pairs were cross correlated during theta only.(ABSTRACT TRUNCATED AT 400 WORDS)