1. Histamine, acting on H(1)-receptors, caused a Ca(2+)-dependent inhibition of forskolin- and isoprenaline-induced cyclic AMP accumulation in monolayers of human U373 MG cells (IC(50) 1.3+/-0.3 microM, maximum inhibition 66+/-3%). The inhibition was not reversed by the protein kinase inhibitor K-252A. 2. Thapsigargin also inhibited cyclic AMP accumulation (IC(50) 6.0+/-0.3 nM, maximum inhibition 72+/-1%). In the absence of extracellular Ca(2+) 5 microM thapsigargin caused only a 12+/-2% inhibition of cyclic AMP accumulation. 3. The inhibitory effect of 100 nM thapsigargin on forskolin-stimulated cyclic AMP accumulation was blocked by La(3+) (best-fit maximum inhibition 81+/-4%, IC(50) 125+/-8 nM). In contrast, the inhibitory action of 10 microM histamine was much less sensitive to reversal by 1 microM La(3+) (33+/-5% reversal, compared with 78+/-6% reversal of the inhibition by thapsigargin measured concurrently). However, in the presence of both thapsigargin and histamine the inhibition of cyclic AMP accumulation was reversed by 1 microM La(3+) to the same extent as the inhibition by thapsigargin alone. 4.++Thapsigargin (5 microM)+1 microM La(3+) caused only a 20+/-1% inhibition of histamine-stimulated phosphoinositide hydrolysis. 5. There was no indication from measurement of intracellular Ca(2+) of any persistent La(3+)-insensitive Ca(2+) entry component activated by histamine. 6. The results provide evidence that Ca(2+) entry is required for the inhibition by histamine and thapsigargin of drug-induced cyclic AMP accumulation in U373 MG astrocytoma cells. The differential sensitivity of the inhibitory action of the two agents to block by La(3+) suggests that more than one pathway of Ca(2+) entry is involved.